bluuebunny/crossref_metadata_2025 huggingface dataset:
A numerical study of the regions of validity of the Kirchhoff and small‐perturbation rough surface scattering models
Authors: Chen M. F. , Fung A. K.
Published: February 2008

The regions of validity of the Kirchhoff and the first‐order small‐perturbation models in rough surface scattering are examined by numerical simulation. The procedure is to generate a one dimensional perfectly conducting random surface on the digital computer, compute the induced current on the surface due to an impinging plane wave by the moment method, and then calculate the far zone backscattered field and power. This procedure is repeated at least forty times and the results averaged to obtain the average scattered power. It is important to note that many so called Kirchhoff models in the literature involve simplifying assumptions in addition to the Kirchhoff approximation. To avoid confusion the Kirchhoff model in this study uses only the Kirchhoff approximation for the surface current. The far zone scattered field and power are evaluated numerically without further simplifying assumptions. Comparisons between the numerical calculations and the models are made for various values of the surface rms height and correlation length both normalized to the incident wave number (denoted by kσ and kL, respectively). By using these two parameters to form a two‐dimensional space the approximate regions of validity are then established. It is found that due to the inclusion of the coherent scattering component the Kirchhoff model continues to provide good agreement with numerical calculations over small angles of incidence when kσ is less than 0.2 and kL < 2.0, a region where the Kirchhoff method was expected to fail. It is also found that the usual requirements of small height and small slope are inadequate to guarantee the validity of the first‐order perturbation model. It is necessary that kL also be small.
Measuring gradients of meteorological properties in elevated layers with a surface‐based Doppler radar
Authors: Gossard Earl E. , Sengupta Nandini
Published: February 2008

The fundamental relationships between atmospheric properties sensed by radars and the mean height gradients of the properties are reviewed, and various assumptions discussed. Especially, the question of the necessity for assumptions about the turbulent Prandtl number and the Richardson number is examined. A new approach based on the covariance equations is compared with the conventional approach using the variance equations. Relationships between fluctuations of conserved passive scalars are examined. Specifically, the correlation between fluctuations in potential temperature and humidity is derived and found to be essentially ±1 if molecular diffusion is ignored, but it is somewhat reduced by differing rates of molecular diffusion. The relationship of dissipation rate of their variances to dissipation rate of their covariance is given. The implications for relating refractive index variance to the mean temperature and humidity gradients are discussed, and the relationship between optical C2N and radio C2N is given in terms of those mean gradients. The theoretical relationships are compared with measurements made on a 300‐m tower and with a 915‐MHz, vertically pointing Doppler radar, and satisfactory agreement between theory and experiment is found except when cloud return dominates the radar backscatter.
The focal plane coherence function of an imaging antenna and its use in measuring and correcting aberrations
Authors: Cornwell T. J. , Napier P. J.
Published: February 2008

Large reflector antennas are typically limited in efficiency and side lobe level by small errors in the reflector profile. Interferometric arrays suffer from an analogous problem, which can be corrected by the “self‐calibration” technique by exploiting redundant sampling of the aperture plane coherence function. We show that by measuring the focal plane coherence function, the aberrations in reflector antennas can be corrected similarly, and true diffraction limited images obtained.
Intensity correlation function for waves of different frequencies propagating through a random medium
Authors: Bhattacharyya A. , Yeh K. C.
Published: February 2008

The fourth‐moment equation for plane waves of two different frequencies propagating through a two‐dimensional, dispersive random medium, is solved numerically using a split step method. In this numerical scheme, the extended medium is replaced by a series of phase screens interspersed with diffraction layers and thus it provides a test for the validity of using a single effective phase screen to describe an extended random medium in the calculation of the intensity cross‐correlation function. The intensity cross spectrum in the multiple scattering regime is calculated for different values of the ratio of the two frequencies under the “frozen flow” assumption, when temporal evolution of the irregularities is absent. Irregularities with either a Gaussian or a power law spectrum are considered. The effect of varying irregularity strength on the normalized cross correlation of intensity fluctuations observed at the same location is also investigated. The two‐frequency intensity space‐time cross‐correlation function is determined for a special case of “nonfrozen” flow where the irregularities have a random drift superimposed on uniform convection.
Modeling F region gravity waves observed during the WAGS campaign: 1. Special event
Authors: Sheen D. R. , Liu C. H.
Published: February 2008

In the Worldwide Atmospheric Gravity Wave Study (WAGS) campaign, the source‐response relationship between the auroral activities and the gravity waves observed in the ionosphere was studied. Ionospheric parameters observed with the incoherent scatter radars at Sondrestrom and Millstone Hill were compared with predicted results based on gravity wave theory. In the observed data, usually two types of disturbances can be identified. One is the distinct, semiperiodic traveling ionospheric disturbance. The other is the ever present, semirandom perturbations. The first type will be classified as the “special event,” and in this paper, we shall concentrate on the “special event” which was observed on October 18, 1985 during a moderately magnetically active period. The observed parameters used in this study are the ionization density and the line‐of‐sight ion velocity. This provides more information than in most of the previous investigations of traveling ionospheric disturbances, in which only electron density perturbations were used. The data are analyzed and compared with theoretical models to yield information about the special gravity wave event, such as the vertical and horizontal wavelength, the horizontal phase and group speeds, and the kinetic energy density carried by the waves. The observed waves at the two radar sites are related to a possible source region in the auroral zone.
An efficient high‐frequency analysis of modal reflection and transmission coefficients for a class of waveguide discontinuities
Authors: Pathak P. H. , Altintas A.
Published: February 2008

A relatively simple and efficient high‐frequency analysis of electromagnetic modal reflection and transmission coefficients for waveguide discontinuities which are formed by joining different waveguide sections is presented. The analysis extends the concept of geometrical theory of diffraction based equivalent edge currents and utilizes it in conjunction with the reciprocity theorem to describe interior (waveguide) scattering effects. It is noted that the previous use of equivalent edge currents was mostly restricted to exterior scattering by edged bodies, and its application to deal with interior scattering was limited to those guide geometries for which image theory could be used effectively to account for the interior wall effects. The present extension allows one to treat more general two‐ and three‐dimensional waveguide geometries provided the waveguide modes and their associated modal rays can be found explicitly. In particular, expressions for two‐dimensional reflection and transmission coefficients are developed, and numerical results are shown for a flanged, semi‐infinite parallel plate waveguide and for the junction between two linearly tapered waveguides. One sample result is also shown for the reflection coefficient of a three‐dimensional open‐ended circular waveguide. Detailed expressions for three‐dimensional waveguide discontinuities are being reported separately.
On the efficient calculation of the incomplete Airy function with application to edge diffraction
Authors: Cwik Tom
Published: February 2008

When the geometrical theory of diffraction (GTD) and its extensions are used to calculate fields across transition regions or in focal areas, various special functions are necessary to create uniform representations. One such transition function is the incomplete Airy function which is applicable when two reflection points are arbitrarily close to each other and the endpoint of the domain considered. An efficient calculation of the incomplete Airy function is presented and shown to equal known asymptotic expansions when its argument is away from the critical point. By using these expansions, uniform field expressions are found which are consistent with the GTD and maintain its computational efficiency.
A method for real height analysis of oblique ionograms
Authors: Reilly M. H. , Kolesar J. D.
Published: February 2008

A method is presented for the conversion of an HF oblique ionogram to an ionospheric density true height profile, within the usual approximations of spherical symmetry and no magnetic field. It is a lamination method for inversion of the equations for group path delay and sounder range, which are treated on equal footing, similarly to an earlier method by Rao. A reentrant procedure has been added to alleviate the problem of valley ambiguity. Use of the method and its clear improvement over other methods, which invoke further approximations, are demonstrated by calculations on oblique ionograms from known, model daytime profiles. Future directions are indicated.
Observation of ionospheric disturbances following a 5‐kt chemical explosion 2. Prolonged anomalies and stratifications in the lower thermosphere after shock passage
Authors: Blanc Elisabeth , Jacobson Abram R.
Published: February 2008

We have observed unusual stratifications and prolonged spectral deformations of radio echoes after the passage through the E region of the shock wave produced by a 5‐kt chemical explosion. Our observations were obtained by vertical HF pulsed ionospheric sounding. At the total reflection level of the sounding waves, at a horizontal distance of 30 km from the explosion, the passage of the shock wave generated a complex structure suggesting a significant deformation of the reflecting surface. This effect was followed at the E region total reflection level (130 km) by a broadening of the Doppler spectra, persisting several tens of minutes. Two spectral peaks appear at 0.5 Hz, suggesting an amplitude modulation with a period of the order of 2 s. At lower virtual heights (94, 103, and 115 km), stratifications appear after the shock passage and move downward at velocities of the order of 23 m/s. These layers persist for several minutes. Irregularities have also been observed in the E region for over 30 min after the shock passage by a network of continuous wave HF ionospheric sounders located 250 km west of the source. The origin of these structures is discussed.
Polar mesosphere summer echoes observed with the EISCAT 933‐MHz radar and the CUPRI 46.9‐MHz radar, their similarity to 224‐MHz radar echoes, and their relation to turbulence and electron density profiles
Authors: Röttger J. , Rietveld M. T. , La Hoz C. , Hall T. , Kelley M. C. , Swartz W. E.
Published: February 2008

During a campaign to study polar mesosphere summer echoes (PMSE) the European Incoherent Scatter (EISCAT) UHF and VHF radars and the Cornell University portable radar interferometer (CUPRI) VHF radar were operated in Tromsö in the summer of 1988. Also, for the first time the EISCAT UHF radar detected coherent echoes from the mesosphere. Their relation to the echoes recorded simultaneously with the CUPRI radar is studied, and it is shown that these 933‐MHz echoes are of the same nature as the VHF radar polar mesosphere summer echoes. Comparisons are made of the CUPRI PMSE with the electron density profile, measured simultaneously with the EISCAT 933‐MHz radar. A narrow electron density depletion seen in the EISCAT data appears to occur at the same altitude as the CUPRI PMSE and is consistent with earlier rocket measurements. It appears from the turbulence energy dissipation rates, which we estimate from these radar measurements, that the electron gradients related to these depletions have to be considerably steep to explain the large radar reflectivities. Doppler spectra of PMSE observed with the EISCAT UHF radar are much narrower than the incoherent scatter spectra and are used to obtain an independent estimate of the turbulence energy dissipation rates. Some possible origins of the scattering process of the polar mesosphere summer echoes observed at VHF and UHF are outlined, such as enhanced electron density fluctuations as well as steep electron density gradients in the presence of cluster ions in the cold polar mesosphere. We believe that it is impossible to explain the large radar reflectivities at both 46.9 and 933 MHz with a single turbulence mixing mechanism.
Measurement of waves on a wire parallel to a strip grating of finite width
Authors: Jacard Benjamin
Published: February 2008

The dispersion characteristics and coupling of modes propagating along a wire parallel to a strip grating of finite width are studied experimentally. The structure supports a radiating fast wave together with a second mode that could be fast or slow, depending on the frequency and the width of the grating. The experimental results agree reasonably well with predictions based on periodically loaded transmission line theory.
Correcting atmospheric‐induced phase errors of a synthetic aperture antenna array by time series modeling and Kalman filtering
Authors: Zheng Yi , Basart John P. , Koh Yong‐seon
Published: February 2008

A new technique is discussed and developed for correcting atmospherically induced errors in phase data collected by radio astronomy interferometers and synthetic aperture antenna arrays. The main features of this technique are modeling and filtering the information content of the complex visibility data in time sequences. Because the atmospheric phase variations are highly correlated in time, they can be described by stochastic time series models. In conjunction with other radio astronomy data processing algorithms, a time series modeling and parameter estimation technique is developed to obtain noise models and source models from observed phase data. These models can be in the form of stochastic difference equations, autocorrelation, power spectral density, and a state space format ready for further data processing. Once the models are in a state space format, the Kalman filter is used for optimally extracting source information from the noisy data. The synthetic image is then improved by reducing the phase error. The quality of the corrected phase data is quantitatively described by the error covariance matrix of the Kalman filter. One useful application of this technique is for reducing atmospherically induced phase errors of small synthesis arrays that have too few antennas to apply self‐calibration. Another application of this technique is for improving the performance of large synthesis arrays when the standard calibration methods are insufficient for correcting very noisy phase data. This technique has been tested using the very large array (VLA) (operated by the National Radio Astronomy Observatory) and the Hat Creek millimeter interferometer.
Subsidence in Late Paleozoic basins in the northern Appalachians
Authors: Bradley Dwight C.
Published: February 2008

During the interval between continental collision in the Devonian and continental breakup in the Triassic the northern Appalachians became the site of a wide plate boundary zone of dominantly right‐lateral strike slip. As is typical of intracontinental transforms, tectonism was both diachronous and rapidly variable along strike through regimes of ‘pure’ strike slip, transpressional deformation, and rapid subsidence of extensional basins. Up to 9 km of mainly nonmarine, clastic sediments accumulated in these local depocenters, which subsided episodically in two stages: (1) an initial phase of stretching and thinning of the lithosphere, when subsidence was rapid, fault controlled, and often accompanied by volcanism and (2) a subsequent phase of gradual thermal subsidence, during which the depositional basins expanded to bury the earlier border faults and progressively younger sedimentary units onlapped basement. The largest depocenter, the Magdalen Basin, opened as a pull‐apart between strike slip faults in Newfoundland and New Brunswick from late Devonian to early Carboniferous. Subsequent thermal subsidence affected a large area during medial and late Carboniferous, a phenomenon that is well recorded to the north and west, where no later tectonism occurred. In areas to the south and east of the basin, strike slip on other faults continued into the time of thermal subsidence, introducing complications such as localized transpressional deformation and rapid subsidence in smaller pull‐aparts.
An Ancient Rift Complex and its relation to contemporary seismicity in the New Madrid Seismic Zone
Authors: Braile L. W. , Keller G. R. , Hinze W. J. , Lidiak E. G.
Published: February 2008

Gravity, magnetic, geologic, and seismicity data have been combined in a seismotectonic analysis of the New Madrid seismic zone. Previous studies have presented evidence for several rift zones in this area (Upper Mississippi embayment), including the Reelfoot rift, a late Precambrian‐early Paleozoic failed arm which extends N‐NE from the ancient continental margin. In this study, we propose that the northern terminus of the Reelfoot rift forms a rift complex which appears to correlate well with the seismicity in the area. This correlation suggests that faults associated with this rift complex are being reactivated in the contemporary stress field (ENE compression). If this interpretation is valid, it represents a seismotectonic model which can be used to predict the extent of future seismicity in the New Madrid seismic zone. The proposed rift complex also provides a coherent model for the tectonic development of this region of the North American midcontinent.
Geology in and of China: A tale of two maps
Authors: Hsü Kenneth J.
Published: February 2008

Two tectonic maps of China published in 1976 and 1980 respectively reflect the different geological philosophies of two leading geologists in that big country. J.S. Lee's geomechanical theory, like Maoism, was an indigenous product valued as a ‘bible’ during the years of the Cultural Revolution in China. T.K. Huang, a student of Emil Argand's, managed to apply his teacher's grand synthesis to interprete the tectonics of his homeland. We are still awaiting a third map, which may show plate‐tectonic elements as map units.
Seismotectonics of southern Iran: The Oman Line
Authors: Kadinsky‐Cade Katharine , Barazangi Muawia
Published: February 2008

Seismotectonic characteristics of the transition area between the Zagros continental collision zone and the Makran ocean‐continent convergence zone (the ‘Oman Line’) are carefully examined. A northeast trending zone of earthquakes terminates the Zagros belt of seismicity just west of the Oman Line. This seismic zone extends northeastward beyond the Main Zagros Thrust and coincides with a surface escarpment clearly visible on LANDSAT imagery. Events with accurately determined depths along this seismic zone occurred in the upper 20 km of the crust and possibly show a shallow (<10°) northeastward dip. East of the Oman Line, the Zendan‐Minab fault system is relatively aseismic. The Oman Line region may be characterized by underthrusting of a wedge of Arabian shelf edge beneath Iranian crust or by an indentation of Arabia into the Iranian crustal block as a promontory. Available seismological and geological data cannot uniquely distinguish between these two possible tectonic settings. The area located to the northwest of the Oman Line region, now a zone of continental collision, appears to have been characterized by an episode of Neogene subduction. A well‐located, intermediate‐depth earthquake occurred in this area to the northeast of the Main Zagros Thrust on November 9, 1970. It was accurately located at 107‐km depth beneath a line of Quaternary volcanoes. Its focal mechanism may indicate downdip tension, with nodal planes that strike closely parallel to the trend of the Zagros arc. Comparison of this event and other neotectonic features in this part of Iran with those in other active convergent zones suggests that a descending oceanic lithosphere beneath the Zagros volcanic arc may still be attached to the colliding Arabian plate.
Continental hypsography
Authors: Harrison C. G. A. , Miskell K. J. , Brass G. W. , Saltzman E. S. , Sloan J. L.
Published: February 2008

We have used average continental elevations for 1‐degree‐square areas to construct detailed continental hypsographic curves. The curves available up to the present have been those prepared by Kossinna, but they suffer from some drawbacks, the most important being that the elevation interval is for the most part 1 km, which does not allow for very detailed work. Consequently, we have given data for each 0.1 km interval of elevation. The availability of the data on tapes allowed us also to make calculations of the average elevation of each continent, an important normalizing parameter when comparing continental hypsographic curves, and to add on the effects of the continental shelves. It has also allowed us to recalculate the amount of continent antipodal to continent and to produce a map of the world showing these areas. We have also used the detailed curves and the amounts of continental flooding during the Neozoic to derive records of sea level variation necessary to explain the amount of flooding for each continent through time.
Phanerozoic addition rates to the continental crust and crustal growth
Authors: Reymer Arthur , Schubert Gerald
Published: February 2008

Phanerozoic addition rates to the continental crust are calculated by using seismic profiles through magmatic arcs to measure the crustal volumes added during the active lifespans of the arcs. Data for 17 arcs give addition rates per kilometer of arc in the range 20 to 40 km³ km−1 Ma−1. From these data we deduce a world‐wide addition rate of 1.65 km³ a−1 after adding other contributions to the formation of the continental crust, e.g., from hot spot volcanism. We infer a subtraction rate, mainly by subducting sediments, of 0.6 km³ a−1 and arrive at a net crustal growth rate of about 1 km³ a−1. Growth of the continental crust is necessary to maintain approximately constant freeboard, because the secular decline in the heat production of the mantle causes the ocean basins to deepen. An equation for the growth of the continents as a function of the decline in terrestrial heat flow yields approximately constant growth rate since the Archean of 0.9 km³ a−1, in good agreement with the above estimate. On the average, Archean growth rates must have been 3 to 4 times the present rate. Island arc growth rates are inadequate to explain the formation of the Arabian‐Nubian Shield and the Archean granite‐greenstone terrain of the Superior Province, and a captured island chain in Oregon. We confirm the oceanic island origin of the Oregon terrain on the basis of the large growth rates of hotspot islands.
A mechanical and thermal model for the evolution of the Williston Basin
Authors: Ahern J. L. , Mrkvicka S. R.
Published: February 2008

The Williston Basin is a large, circular, intracratonic basin containing over 4 km of sediments ranging in age from Cambrian to Tertiary. Data on seven formations from over 1300 wells in Montana, South Dakota, and North Dakota were used to determine the shape of the basin over time. By assuming that the lithosphere behaves like an elastic plate on a fluid substrate, the shape of the basin could be closely matched using a point load. The match was improved by adding the equation of a plane to the equation for elastic deformation before fitting the model to the data. The planar adjustment allows for the effects of regional sedimentation or erosion, or eustatic sea level changes. The mechanical modeling indicates that the elastic strength of the lithosphere in this area has generally increased from the Ordovician to the present. The geometrical center of the basin migrated less than 20 km over 450 m.y. The direction and magnitude of tilt of the plane removed from the data is related to tectonic events in western North America. The increasing deflection of the basin over time can be explained by a simple thermal model (which would explain the thickening of the lithosphere) consisting of conductive cooling of a hot region in the lithosphere. Depths to formations in a single well differ from the subsidence predicted by the elastic model with regional tilt removed because of global sedimentation events and the development, in the Mesozoic, of a foreland thrust basin of which the Williston Basin became part. The mechanical and thermal models constrain possible initiating mechanisms for this basin. The thermal modeling and the absence of volcanics in the stratigraphic column rule out a shallow (crustal) heat source. Neither wrench faulting nor crustal stretching were likely to have been significant in the formation of the Williston Basin.
Subduction of young oceanic lithosphere and extensional orogeny in southwestern North America during Mid‐Tertiary time
Authors: Elston Wolfgang E.
Published: February 2008

An “extensional orogeny” deformed the Basin and Range province, probably beginning in the late Eocene (about 40 ± 3 Ma). Its characteristics include partial melting of the continental lithosphere during the “ignimbrite flareup,” massive ductile extension (including detachment faulting), and rise of metamorphic core complexes. The affected zone became about 1200 km wide, possibly double its original width. It rose an average of 1–2 km, despite crustal thinning. Locally, some of the highest mountains of North America, up to 4.3 km high, rose through resurgence of ignimbrite cauldrons and isostatic uplift of underlying plutons. The climax of extension occurred prior to the development of the present basin and range topography. Modeling of major and trace elements and Sr and Pb isotopes strongly suggests that mid‐Tertiary volcanic magmas equilibrated, and probably originated, in the continental lithosphere. Components attributable to subducted oceanic lithosphere have not yet been identified. The rocks seem to belong to two provinces, separated by the quartz diorite boundary line of Moore (1959), which also marks the western limit of North America at the end of the late Paleozoic Sonoman orogeny. To the west, low‐K rocks rest on a basement of predominantly oceanic accreted terranes; to the east, high‐K rocks rest on an autochthonous sialic basement. Within the high‐K province, potassium variations can be correlated with crustal thickness; there is no need to invoke a K‐h relationship. Conventional models of plate convergence and back arc extension which involve subduction of old, rigid, cool, and dense oceanic lithosphere may not apply to the mid‐Tertiary Basin and Range province. The overridden Farallon plate is more likely to have been young, hot, ductile, buoyant, and no denser than continental asthenosphere, having been generated in a spreading center close to North America. Under these conditions, motion of the subducting plate slows and slab‐pull is likely to approach zero. Even prior to ridge‐trench collision, overridden oceanic lithosphere may have become underplated beneath the continental lithosphere and ruptured by rising mantle diapirs. Subducted oceanic lithosphere no longer acted as a heat sink, which could partly account for the great width of the affected zone and the anomalous thermal gradients required for partial melting, extension, and metamorphism. Had these processes not died down, after ridge‐trench collision, the western segment of the Cordillera might have separated from North America to form a Japanlike archipelago, while the Basin and Range province foundered into an analog to the Sea of Japan. Instead of rupturing completely, the Basin and Range province fractured into fault blocks.
Geological constraints, structural evolution, and deep geology of the northwest Scottish Caledonides
Authors: Butler R. W. H. , Coward M. P.
Published: February 2008

The front of the Caledonian orogenic belt in NW Scotland is marked by a zone of foreland thrusting which contains imbricates of Lewisian basement, Proterozoic cover and Cambrian shelf sediments. Faults within this Moine thrust belt, in most localities, propagated in a foreland directed sequence. Therefore the earliest of these thrusts was the Moine thrust, which carried a basement assemblage of Moine and Lewisian rocks over the Cambrian shelf. The development of large duplex and imbricate structures in the footwall postadted movement on this thrust. Hence a restoration of these imbricates provides a minimum estimate of the eastern extent of the Cambrian shelf. No synsedimentary or prethrust extensional features are recognised within the Cambrian succession which displays a remarkable stratigraphic and thickness consistency. This implies a corresponding consistency in crustal structure beneath these cover rocks immediately prior to Caledonian thrusting. A balanced cross section constructed across the northern part of the Moine thrust belt has given a restored width for the Cambrian shelf of 54 km. Geophysical data suggest that the present foreland crustal thickness is 28 km. If this is taken to be the crustal thickness beneath the Cambrian shelf during Caledonian thrusting, a minimum cross sectional area of 1512 km² of Lewisian rocks must remain beneath the present outcrop of the Moine thrust sheet. This foreland basement wedge may have been imbricated subsequent to the development of the Cambrian imbricates. Caledonian structures within the Moine thrust sheet are interpreted as forming an imbricate stack with the Moine thrust acting as a floor thrust. The ductile imbricate thrusts possibly roofed into the Naver slide. Late open folds, which warp foliation, slides, and metamorphic isograds within the Moine sheet, may represent culminations in the foreland Lewisian imbricates beneath the Moine thrust. These postulated culminations may have been sufficient to initiate gravity‐driven structures. This may explain late movements on high‐level faults within the Moine thrust belt.
The development of the SW Pacific Margin of Gondwana: Correlations between the Rangitata and New England Orogens
Authors: Cawood Peter A.
Published: February 2008

Prior to formation of the Tasman Sea, the Late Paleozoic‐Mesozoic Rangitata Orogen of New Zealand and New Caledonia abutted the Paleozoic New England Orogen of eastern Australia. Comparison of the record of Permian‐Cretaceous igneous and deformational events from the two orogens suggests that their tectonic evolution was interrelated and is a consequence of convergent plate interaction along the southwest Pacific margin of Gondwana. The following relations are proposed: (1) termination of arc volcanism and widespread sedimentation in New England, together with the onset of regional deformation and crustal anatexis were synchronous with the commencement of volcanism and sedimentation within the Rangitata Orogen; (2) Early Permian andesitic volcanism in eastern New England represents an along‐strike extension of the Brook Street terrane of New Zealand; (3) Late Permian regional deformation in New England coincides with both a break in subduction‐related igneous activity in the New England and Rangitata Orogens and a shift in the locus of this activity; (4) Late Permian‐Triassic calc‐alkaline igneous activity in New England correlates with a phase of relatively continuous accumulation of pyroclastic material in the forearc basin of the Rangitata Orogen; (5) cessation of plutonism in New England corresponds with commencement of formation of the Esk Head Melange in New Zealand and the probable commencement of juxtaposition of the Te Anau and Alpine Assemblage; (6) Late Cretaceous epizonal plutons intruded into the New England Orogen are similar in character and age to those emplaced during the final phases of Rangitata orogenesis, and both appear to mark initial stages of rifting associated with formation of the Tasman Sea. The generation of Permian and Triassic igneous activity in eastern New England by convergent plate interaction results, on present reconstructions of the Gondwana margin, in an excessively wide arc‐trench gap succession, for the position of the trench is constrained to east of New Caledonia from the Permian onward. This suggests there may have been some rearrangement of tectonic elements along the margin resulting in widening of the Lord Howe Rise between Australia and New Caledonia in the period between termination of igneous activity in New England and formation of the Tasman Sea in the Late Cretaceous.
Propagating rifts: Crustal extension during continental rifting
Authors: Martin A. K.
Published: February 2008

Propagating rifts may not be incompatible with plate tectonic tenets which are considered at the appropriate scale. Propagation of a rift is a natural consequence of relative motion about a pole of rotation. It is not necessarily valid that (1) rifts propagate in one direction or (2) crustal stretching increases with progressive propagation of a rift.
Tectonic implications of subthrust structures revealed by seismic profiling of Appalachian‐Ouachita Orogenic Belt
Authors: Lillie Robert J.
Published: February 2008

A growing body of deep seismic reflection profiles across the Appalachian‐Ouachita orogenic belt reveals subthrust structures which have been previously interpreted as products of continental rifting, of later thrusting, or of some combination of rifting and thrusting. The subthrust features can be placed into three general categories: (1) small normal faults which offset lower plate shelf sequences and basement in the foreland and have been attributed to thrust loading, perhaps reactivating earlier rifting faults, (2) moderately dipping seismic events which lie entirely below lower plate shelf sediments in the foreland, interpreted in part as strata deposited during the earlier rifting, and (3) hinterland‐dipping reflection sequences beneath more interior portions of the mountain belt, previously interpreted as thrust faults which imbricate sediments and basement related to the earlier rifted margin. These interpretations are evaluated through a comparison of reflection profiles across the orogenic belt to previously interpreted seismic lines across modern extensional and compressional settings. The comparisons suggest that although some normal fault offsets of lower plate shelf sequences may be original or reactivated rifting faults, the dipping events entirely beneath the shelf sequence are more diagnostic of the rifting process. These dipping sequences are therefore consistent with previous interpretations that large portions of the earlier rifted margin are preserved in an undeformed state beneath thrust sheets in the Appalachian foreland. The comparisons also suggest an alternative to interpretations of the hinterland‐dipping sequences as thrust‐related, in that the sequences are similar in seismic appearance to that of volcanic wedges commonly observed in the narrow zone separating continental from oceanic basement on modern passive margins. The implication of this new interpretation is that the actual continent/ocean boundary related to the earlier rifted margin may be preserved on the lower thrust plate beneath interior portions of the Appalachians and Ouachitas.
Middle to Late Jurassic tectonic evolution of the Klamath Mountains, California‐Oregon
Authors: Harper Gregory D. , Wright James E.
Published: February 2008

The geochronology, stratigraphy, and spatial relationships of Middle and Late Jurassic terranes of the Klamath Mountains strongly suggest that they were formed in a single west‐facing magmatic arc built upon older accreted terranes. A Middle Jurassic arc complex is represented by the volcanic rocks of the western Hayfork terrane and consanguineous dioritic to peridotitic plutons. New U/Pb zircon dates indicate that the Middle Jurassic plutonic belt was active from 159 to 174 Ma and is much more extensive than previously thought. This plutonic belt became inactive just as the 157 Ma Josephine ophiolite, which lies west and structurally below the Middle Jurassic arc, was generated. Late Jurassic volcanic and plutonic arc rocks (Rogue Formation and Chetco intrusive complex) lie outboard and structurally beneath the Josephine ophiolite; U/Pb and K/Ar age data indicate that this arc complex is coeval with the Josephine ophiolite. Both the Late Jurassic arc complex and the Josephine ophiolite are overlain by the “Galice Formation,” a Late Jurassic flysch sequence, and are intruded by 150 Ma dikes and sills. The following tectonic model is presented that accounts for the age and distribution of these terranes: a Middle Jurassic arc built on older accreted terranes undergoes rifting at 160 Ma, resulting in formation of a remnant arc/back‐arc basin/island arc triad. This system collapsed during the Late Jurassic Nevadan Orogeny (150 Ma) and was strongly deformed and stacked into a series of east‐dipping thrust sheets. Arc magmatism was active both before and after the Nevadan Orogeny, but virtually ceased at 140 Ma.
Late Caledonian sinistral displacements in Britain: Implications for a three‐plate collision model
Authors: Soper N. J. , Hutton D. H. W.
Published: February 2008

In recent years, most geotectonic syntheses of the North Atlantic Caledonides have adopted a two plate configuration, with a roughly E‐W closure direction between Laurentia and Baltica producing the N‐S striking Scandinavian and East Greenland Caledonides and inducing dextral strike‐slip along the NE‐SW oriented British sector of the Iapetus suture. The North German‐Polish Caledonides are a third arm to the Appalachian‐N. Atlantic Caledonide orogen. Recently, Ziegler has proposed that the mid‐Palaeozoic deformation belts of S. Britain ‐ N. Germany‐Poland and also of central Europe were produced by the northward impact of microcontinental fragments onto the southern margin of the already sutured continental mass of Laurentia‐Baltica. Some ascribe the whole Appalachian orogen to a sequence of terrane accretion events in mid‐Ordovician through mid‐Devonian time. A N‐S collision direction would induce sinistral shear across the British sector of the Iapetus suture. In this paper, we outline the evidence for sinistral displacements within the British Caledonides and for their age. We conclude that the “non‐metamorphic Caledonides” were produced by the northerly accretion of a Cadomian terrane in late Silurian‐early Devonian time, and that this event was entirely separate from the earlier Laurentia‐Baltica collision which produced the post‐Grampian thrust related “metamorphic Caledonides” of Scotland.
Alleghenian reconstruction and subsequent evolution of the Gulf of Mexico, Bahamas, and Proto‐Caribbean
Authors: Pindell James L.
Published: February 2008

A detailed model for the evolution of the Gulf of Mexico, the Bahamas and the Proto‐Caribbean is built within the framework provided by a detailed initial Alleghenian (western Pangean) reconstruction and an accurate subsequent relative‐motion history between North America and Gondwana (northern Africa and South America). The Alleghenian reconstruction closes all pre‐Jurassic oceans; accounts for Jurassic attenuation of continental crust by restoring that attenuation to original prerift continental thicknesses; incorporates an improved Equatorial Atlantic fit between northern Brazil and the Guinea margin of Africa; quantitatively removes changes in shape of northern South America due to Late Cretaceous and Cenozoic accretion and internal deformation; includes pre‐Mesozoic continental crust presently underlying the western Bahamas and southern Florida; and correlates Late Paleozoic geology of Yucatan with its neighboring continental masses. Extension occurred within the Gulf of Mexico from Late Triassic to earliest Cretaceous time, but seafloor spreading was delayed until the Late Callovian. This divided a single Gulf‐wide salt basin into the Louann and Campeche salt provinces. The Yucatan block progressively rotated about 43 degrees counterclockwise away from the Texas‐Louisiana margin around a pole in northern Florida. The Tamaulipas‐Golden Lane‐Chiapas fault zone of eastern Mexico is interpreted as the remains of an initially intracontinental transform system along which Yucatan migrated. Attenuated continental crust beneath southern Florida and the western Bahamas, termed here the Florida Straits block, migrated approximately 300 km out of the eastern Gulf, approximately along Central Atlantic flow lines. These rotations are consistent with recently suggested magnetic anomaly trends in the Gulf of Mexico (Shepherd et al., 1982; S. Hall, personal communication, 1984). The Proto‐Caribbean formed synchronously by a fan‐like rotation of Yucatan away from Venezuela.
Neogene block tectonics of eastern Turkey and northern South America: Continental applications of the finite difference method
Authors: Dewey J. F. , Pindell J. L.
Published: February 2008

Continental plate boundary zones are, generally, wide diffuse zones of deformation within which, in the upper 5 to 15 km, may be recognized blocks, which are bounded by fault zones within which strain is highly concentrated and along which slip rates are fairly high (2–30 m Ma−1). These blocks may be irregular flakes, defined by older crustal inhomogeneities of an upper brittle crust below which strain occurs in a ductile and more homogeneous lower crust. With examples from the convergent plate boundary zone of eastern Turkey and the southern Caribbean plate boundary zone of northern South America, we use plate slip rates and trends and slip rates of faults to construct block vector diagrams to deduce the sums of relative motion among block mosaics, slip rates and trends of ill defined block boundaries, and the extent to which blocks are internally strained.
Geometry of Cenozoic extensional faulting: Dixie Valley, Nevada
Authors: Okaya David A. , Thompson George A.
Published: February 2008

Precise definition of geometric relationships between individual basins and ranges may help to reveal the mechanical processes of Basin and Range Cenozoic extensional faulting at depth. Previous studies have attempted to identify simple horsts and grabens, tilted crustal blocks with planar faulting, or tilted crustal blocks with listric faulting in the shallow crust. Normal faults defining these crustal blocks may root (1) individually in the ductile lower crust, (2) in regional or local low‐angle detachment faults, or (3) in igneous intrusions or decoupling surfaces produced by the intrusions. The present study, in Dixie Valley, west‐central Nevada, makes use of a seismic reflection survey, gravity models, seismograms from earthquakes occurring on December 16, 1954, and geometrical block models. These data show a structurally asymmetric basin bounded by a single zone of faulting on the northwest and by a downbowed and step‐faulted floor to the southeast. The northwest bounding fault is moderately dipping (50°) and planar to a depth of 3 km. The southeast boundary is step‐faulted, and altogether the faults indicate an extension of 20% across the valley at the rate of 0.38 mm/y for the last 8 my. Synthetic earthquake seismograms confirm a focal depth of 15 km and fault dip of 62° for the Fairview Peak earthquake and suggest that the focal depth of the Dixie Valley earthquake was also 15 km instead of the previously reported 40 km. Local microearthquakes cluster around 10–15 km. The geometrical block models indicate that crustal horst‐graben faulting and planar, high‐angle normal faults rooted in a low‐angle detachment surface do not readily account for development of the subsidiary (step) faults found in Dixie Valley. Extension of the crust by intrusion may develop high‐angle faults and, with further intrusion, may develop the subsidiary faults and produce a complex, sagged, asymmetric graben like Dixie Valley.
Structural study of intra‐oceanic thrusting in the KLamath Mountains, northern California: Implications on accretion geometry
Authors: Cannat Mathilde , Boudier Françoise
Published: February 2008

The kinematics of the basal intra‐oceanic thrust of the Trinity, Preston Peak, Seiad and Josephine ultramafic bodies of the Klamath Mountains are deduced from microstructural and mineral preferred orientation studies. When corrected of possible postthrusting rotations, these kinematics are thought to reflect the paleoconvergence geometry of the Pacific plate relative to the North American continent at the time of intra‐oceanic thrusting. We first detail the kinematics of thrusting for each studied massif. These results are then integrated in a plate tectonics reconstruction, and a model of continuous NE‐SW convergence for Mesozoic times is proposed.
a Paleozoic suture in eastern Gondwanaland
Authors: Baillie Peter W.
Published: February 2008

A new tectonic interpretation of southeastern Australia, Tasmania, and north Victoria Land, Antarctica, is given, in which a major N‐S trending lineament, representing a suture zone, is recognized separating essentially shallow water shelf and continental deposits of latest Cambrian or Early Ordovician age from deeper water Ordovician flysch deposits. Lateral offset of this lineament, which is interpreted as a suture along which thrust telescoping and major strike‐slip faulting occurred, could adequately explain the present distribution of major rock types in the three areas and also removes a number of problems previously encountered in correlation.