bluuebunny/crossref_metadata_2025 huggingface dataset:
Rise and fall of directed transient: Use of Mellin transformation in time domain problems
Authors: Lee Hung‐Mou
Published: February 2008

The Mellin transform is utilized in the study of the time domain radiation from a circular disk. In the Mellin transformed space, the integrals over the source distribution for the frequency domain fields and the Fourier transform of the fields to the time domain can be carried out in closed form. The source is a uniform distribution of current which is switched on at t = 0 and varies as (t/T)2δ for some positive T. It is found that δ must be positive when the source is assumed to be switched on at t = 0. Furthermore, in some finite volume of space, the transient pulse is plane wave‐like and propagates along the axis of the disk. Its E and H field strengths decrease as R−2δ. Its energy decreases as R−1–4δ. A measure of the effectiveness of the current source in generating such a directed transient pulse is also provided.
Relations between electric and magnetic polarizabilities and other related quantities
Authors: Lee K. S. H.
Published: February 2008

The magnetic polarizability tensor, the demagnetization tensor, and the depolarization tensor of an ellipsoid are all related to its electric polarizability tensor. The four tensors are completely described by two scalars. The relationship between the electric and magnetic polarizabilities of an ellipsoid can be applied directly to an elliptic aperture. The total cross section of an antenna integrated over wavelength is given by the antenna's polarizabilities.
Cross‐spectral analysis of spaced‐receiver measurements
Authors: Costa Emanoel , Fougere Paul F.
Published: February 2008

The apparent velocity calculated by the dispersion analysis is interpreted in terms of the fundamental parameters of the cross‐correlation analysis of spaced‐receiver data, without any assumptions in addition to those already present in the latter technique. The dispersion analysis is then generalized, by assuming a realistic (power law) functional form for the power spectral density of the irregularities in the ionospheric F region. Next, an algorithm is proposed to estimate the anisotropy and the true drift velocity of the ground diffraction pattern directly from the cross spectra of the received signals. This technique is applied to HiLat spaced‐receiver data recorded at Tromsø, Norway (69.7°N, 18.9°N), in two different modes. In the first, it is applied on a relatively broad frequency band and its results compared with those from the cross‐correlation analysis. A good general agreement is found. In the second mode, the technique is repeatedly applied on narrow and adjacent frequency bands, to detect possible frequency variations of the anisotropy of the diffraction pattern. A behavior analogous to that of the apparent velocity, previously reported in the literature, is then observed.
New ionospheric and magnetospheric specification models
Authors: Tascione Thomas F. , Kroehl Herbert W. , Creiger Roderick , Freeman John W. , Wolf Richard A. , Spiro Robert W. , Hilmer Robert V. , Shade John W. , Hausman Bonnie A.
Published: February 2008

This paper presents information on two new models: an ionospheric specification model and a magnetospheric specification model. The goal of these models is to provide operational information based on real‐time satellite and ground‐based measurements. The ionospheric model will be described in detail and the magnetospheric model, which is in an earlier state of development, will be outlined.
A comparison of equatorial electron content in the Indian and American longitudes
Authors: Klobuchar J. A. , Rastogi R. G.
Published: February 2008

Total electron content (TEC) measurements taken at the magnetically equatorial stations located at Ootacamund, India and Huancayo, Peru by the group delay technique from radio beacon signals transmitted from the ATS 6 geostationary satellite show excellent agreement, though these stations are at widely different longitudes and are at nearly opposite geographic latitudes. Data from both stations were taken during the solar minimum period 1975–1976. The equivalent slab thickness, the ratio TEC/Nmax, also indicated similar F region profile shape in the two longitude sectors. The standard deviation of equatorial daytime TEC is significantly smaller than at other latitudes.
Polar cap plasma convection measurements and their relevance to the modeling of the high‐latitude ionosphere
Authors: Buchau Jurgen , Reinisch Bodo W. , Anderson David N. , Weber Edward J. , Dozois Claude
Published: February 2008

Plasma convection measurements, using Digisonde ionospheric sounders, have been conducted in the central polar cap at Thule, Greenland (86° CGL) and more recently at Qaanaaq, Greenland (87° CGL), in the auroral oval at the Air Force Geophysics Laboratory Goose Bay Ionospheric Observatory (65° CGL), and at suboval latitudes at Argentia NAS (57° CGL). The plasma convection or ionospheric drift measurements conducted at Thule and Qaanaaq during campaigns from the 1983–1984 winter to the present provide evidence that antisunward convection dominates in the polar cap with velocities typically between 300 and 900 m s−1. Velocity reversals or shears were observed in association with polar cap F layer auroras during quiet magnetic conditions. Observations of the plasma drift at Goose Bay show, as expected, a drift reversal from westward to eastward around midnight CGLT, indicating the rotation of Goose Bay from the evening into the morning convection cell. Observations at Argentia, typically a suboval/trough station, provide evidence under magnetically disturbed conditions for the midnight reversal of the antisunward flow pattern. However, the data at Argentia are generally less consistent under magnetically quiet conditions. This likely indicates the proximity of Argentia to the boundary between corotating and convecting plasma. Recent theoretical calculations of electron density profiles within the high‐latitude/polar cap ionosphere demonstrate that the diurnal f0 F2 variation observed at Thule is controlled by the plasma convection pattern and the associated drift velocities. The model calculations for Bz < 0 and Bz ≈ 0 show factors of 2 to 3 differences in Nmax over Thule, supporting the stated importance of convection pattern and velocity measurements for the modeling of the high‐latitude ionosphere.
The real height analysis of ionograms: A generalized formulation
Authors: Titheridge J. E.
Published: February 2008

A new procedure is described for the calculation of real height profiles from ionospheric h′(ƒ) records. The equations are formulated in a way which can reproduce all current methods of analysis, and provide many powerful extensions. For general use a sixth‐order overlapping polynomial analysis is recommended, with real height sections determined by a least squares fit to real and virtual height data. Coefficients can also be obtained for single polynomials fitted separately to each ionospheric layer. All methods include Chapman layer peaks, and a model valley between layers. High‐order least squares calculations give greater accuracy and stability than any current procedures, and are particularly suitable for the analysis of high‐density data from digital ionosondes.
An observation of atmospheric gravity wave cause and effect during the October 1985 WAGS campaign
Authors: Rice D. D. , Hunsucker R. D. , Lanzerotti L. J. , Crowley G. , Williams P. J. S. , Craven J. D. , Frank L.
Published: February 2008

On October 18, 1985, moderate geomagnetic activity (Kp = 4+) near 1200 UT was followed by observations of a large‐scale traveling ionospheric disturbance (TID) at observatories in Greenland, eastern North America, and Europe. Estimates of the speed and direction of the TID indicate that it was caused by an atmospheric gravity wave expanding from a localized source over the Arctic Ocean north of eastern Siberia. Auroral imaging shows that the source region was located near the westward edge of an expanding auroral bulge and may have been associated with a westward traveling surge (WTS). The largest TID amplitudes were observed along a line perpendicular to the orientation of the auroral oval, and TID periods increased with distance from the source region. This set of observations provides one of the most complete cause and effect relationships found thus far in the study of atmospheric gravity waves.
On the completeness of polarization diversity measurements
Authors: McCormick G. C.
Published: February 2008

Polarization measurements are deemed complete when measurement at an additional polarization yields no new information. This condition is satisfied, in completely polarized scatter, by measurements of the four elements of the scattering matrix; this can be accomplished by transmitting a particular polarization and its orthogonal. The corresponding measurements for partially polarized radiation are the elements of the coherency matrix. This matrix does not yield diagonal product terms, nor are these terms redundant. They are readily obtained, however, by the transmission of a third polarization. Examples relating circular and linear polarizations are given.
Measurement and modeling of millimeter‐wave scattering from tree foliage
Authors: Ulaby F. T. , Haddock T. H. , Kuga Y.
Published: February 2008

Because the constituent elements of a tree canopy, namely the leaves, needles, branches, and trunks, have complex geometries with curvatures and surface roughness scales that are comparable to or larger than the wavelength at millimeter wavelengths, the traditional approach used to compute the phase function of the vegetation volume is totally impractical. In this paper we propose a relatively simple model for characterizing the phase function on the basis of direct experimental data. The model is used in conjunction with a solution of the radiative transfer equation to predict the backscattering behavior of tree canopies. The model is found to provide very good agreement with radar observations made at 35, 94, and 140 GHz.
Derivation of generalized transition/boundary conditions for planar multiple‐layer structures
Authors: Ricoy M. A. , Volakis J. L.
Published: February 2008

Infinite‐order generalized impedance boundary conditions (GIBCs) and generalized sheet transition conditions (GSTCs) for planar multilayer configurations are developed via the Taylor series expansion method. The conditions are derived in a matrix product form where each matrix corresponds to a specific layer. An overall composite boundary/transition condition is obtained by making finite‐order approximations to the elements of each matrix for the cases of “low‐contrast” and “high‐contrast” material layers. The accuracy of the truncated boundary conditions is examined by comparing their implied reflection and transmission coefficients with the corresponding exact coefficients. Design curves are also given which relate the maximum order of the conditions required to simulate a coating or layer of specific thickness and contrast. Expressions are then derived for the reflection and transmission coefficients of the GIBC/GSTC sheets, and these are compared to exact coefficients to demonstrate the validity of the derived GIBCs/GSTCs.
MU radar: New capabilities and system calibrations
Authors: Fukao Shoichiro , Sato Toru , Tsuda Toshitaka , Yamamoto Mamoru , Yamanaka Manabu D. , Kato Susumu
Published: February 2008

The middle and upper atmosphere (MU) radar of Japan is a unique mesosphere‐stratosphere‐troposphere type radar with an active phased array system. The MU radar has proved so reliable that continuous trouble‐free operations over many days are possible. A brief description of new capabilities recently implemented and calibrations made to confirm the performance of the system are presented herein.
Modeling the F2 layer peak height in terms of atmospheric pressure
Authors: Rishbeth H. , Edwards R.
Published: February 2008

Ionospheric data from several stations are used to verify that, at any given local time, the mid‐latitude F2 peak tends to lie at fixed values of atmospheric pressure, as expected from theory. The pressure values are found empirically to be about 2 × 10−10 atmosphere at noon, 2 × 10−11 atmosphere at midnight. Given that the global Mass Spectrometer/Incoherent Scatter (MSIS) model of the neutral thermosphere is now readily available, we suggest that the “constant pressure” approach provides a simple way of modeling the height hmF2 for practical purposes, for a wide range of solar and geomagnetic activity, with better consistency than is given by methods based on ionosonde data alone.
Riometer quiet day curves determined by the maximum density method
Authors: Drevin G. R. , Stoker P. H.
Published: February 2008

Riometer recordings of cosmic radio noise in polar regions show a variability in sidereal variation during subsequent quiet days. The fitting of a low‐order Fourier series to maximum density points in sidereal time intervals has the advantage that as few as 7 days may be used to determine a quiet day curve. Quiet day curves determined for periods of 7 days in a sliding sequence show a short‐term variability of about 0.13 dB in signal strength. For periods of 30 days this variability is 10 times less. Absorption events should be obtained from the quiet day curve appropriate for the period of the event. Absorption obtained from quiet day curves determined for maximum density points will be about 0.05 dB less for 30 MHz than using the inflection point method.
Ionospheric RF lidar
Authors: Fukuchi T. , Wong A. Y. , Wuerker R. F.
Published: February 2008

A new diagnostic consisting of a high‐power radio frequency (RF) or microwave transmitter and a ground‐based lidar (light detection and ranging) system is proposed for probing the ionosphere at heights of 80–200 km. The high‐power transmitter creates energetic electrons in the ionosphere, which excite molecules to higher energy levels. These excited molecules become targets for a laser ranging system by resonantly absorbing and reradiating light at specific wavelengths. A laser pulse tuned to a specific transition wavelength is fired from a ground‐based laser, and the reradiated light is detected by a ground‐based light collector. A study of atmospheric species for ranging was performed, and the most suitable species were found to be N2 and N2+. A laser whose output is matched exactly to the vibrational‐rotational spectrum of ionospheric N2 is proposed as the lidar master oscillator instead of a tunable dye laser.
On the efficiency of ionospheric ELF generation
Authors: Papadopoulos K. , Chang C. L. , Vitello P. , Drobot A.
Published: February 2008

The scaling laws that control the efficiency of converting ground‐based HF power to ELF power by using modulation of the polar electrojet current is discussed. The analysis is based on kinetic calculations of the modification of the ionospheric conductivity by HF waves in conjunction with the experimental results reported from the Tromso Max Planck and the Alaska high power auroral simulation (HIPAS) facilities. It is shown that the efficiency can be increased by more than a factor of 104 by (1) using phasing to sweep the antenna beam over an area spanned by a maximum tilt of 35°, on a time scale faster than the cooling rate at the heating altitude (<100 μs) and (2) localization of the heating at the E region (90–100 km) where electron runaway can be induced resulting in substantial modification of the Pederson conductivity.
Beat heating of the ionosphere with high‐power rf pumps
Authors: Katsouleas T.
Published: February 2008

This report examines the potential for heating a layer of the ionosphere by beating two high‐power RF pumps at the local plasma frequency. Simple scaling laws for the heating efficiency and final temperature of the layer as functions of the pump intensities and frequencies are found for both copropagating and counterpropagating pumps. The results are compared to single frequency heating by collisional absorption (inverse bremstrahlung) near the resonance layer.
Channel probe observations of the auroral ionosphere during high‐power auroral stimulation heating experiments
Authors: Wagner L. S. , Goldstein J. A. , Lind R. W. , Wong A. Y. , McCarrick M. J.
Published: February 2008

Perturbations of the bottomside auroral ionosphere caused by a high‐power HF heater have been observed using a bistatically configured ionospheric channel probe. The heater transmitter power and antenna directivity resulted in a power density of the order of 1 mw/m2atElayer heights and 0.2 mw/m2atFlayer heights.Omode polarization was used for heating at a frequency of 2.85 MHz. Effects were observed at bothEandFlayer heights. TheFlayer effects consisted of enhanced attenuation, modified group delay at frequencies above and below the heater frequency, and a change in the character of the received signal from that of a time stable specular return to a time variable irregular return. All of these effects were most pronounced at frequencies closest to the heater frequency and diminished as the frequency deviated from the heater frequency. The formation of a localized heated cavity in theFlayer is thought to be responsible for the observed changes inFlayer signal group delay. Some form of parametric decay instability is thought to be responsible for the irregular character of the return signal during heating and for the enhanced attenuation which primarily affected theOmode of the probing signal. Heating induced Doppler effects were also noted in a second experiment. In particular, the main Doppler component exhibited a sharp change with the onset of heating followed by a steady change in Doppler from an initially large positive value to a large negative value at the time of heater shutdown. After heater shutdown the Doppler frequency relaxed to a steady value which differed from its preheating value. The spread of the Doppler return was also observed to broaden during heating which is consistent with the irregular character of the return during heating. TheElayer return consisted of normalElayer echoes at frequencies below 2.5 MHz and of weak slant sporadicEechoes at and above 2.5 MHz. The slant sporadicEechoes were strongly attenuated during heating. An additionalElayer effect involving what appears to be a localized, nonblanketing enhancement of ƒ0Ewas observed during heating. The most likely explanation for the enhanced attenuation of the slant sporadicEisElayer heating caused by energetic electron precipitation from the heatedFlayer. The enhanced ƒ0Ecould also be attributed to high‐energy electron precipitation from theFlayer but direct ohmic heating of theElayer is an equally plausible explanation.
Evidence for a major Late Precambrian tectonic event (rifting?) in the Eastern Midcontinent Region, United States
Authors: Keller G. R. , Bland A. E. , Greenberg J. K.
Published: February 2008

Recently acquired gravity and aeromagnetic data delineate a large linear gravity anomaly which extends through eastern Kentucky and Tennessee and coincides with a zone of complex, high‐amplitude magnetic anomalies. Basement lithologies in the area can be interpreted as a bimodal volcanic suite which is locally peralkaline in nature. These volcanics appear to be metamorphosed where they lie east of the Grenville front, suggesting they predate the Grenville metamorphic event. The available gravity, aeromagnetic, seismic refraction, and petrologic data, along with regional correlations, suggest that the best tectonic interpretation of these data is that a Keweenawan rift zone extended through the area. This rift can be roughly outlined by the gravity high, which is locally offset, suggesting the presence of transform faults. The boundaries of this rift have been locally reactivated and, in fact, a recent earthquake was located along its western boundary in northern Kentucky.
The subduction of the Farallon Plate beneath North America as derived from relative plate motions
Authors: Jurdy Donna M.
Published: February 2008

High rates of subduction and near normal convergence of the Farallon plate correlate with the duration of the Laramide Orogeny in the Western Cordillera. The motion of the Farallon plate relative to North America is reconstructed back to the late Mesozoic using relative plate motion data. This reconstruction involves the use of a plate circuit in which each plate is linked to another with seafloor spreading information in order to relate converging plates. Two models for relative motions are considered. The preferred model predicts an increase in the subduction rate of the Farallon plate, exceeding 150 mm/yr, with a direction almost due east and perpendicular to the continental margin. The duration of this nonoblique (normal), high‐rate subduction correlates well with the duration of the Laramide Orogeny. This correlation must be regarded as only suggestive; the probable errors, when combined and propagated through the relative motion circuit, may be of the order of the effect observed. However, it is encouraging that these reconstructed Farallon/North American relative motions are similar to the published predictions of others, using the alternative, more direct approach of hot spot reconstructions.
Cenozoic plate motions and the volcano‐tectonic evolution of western Oregon and Washington
Authors: Wells R. E. , Engebretson D. C. , Snavely P. D. , Coe R. S.
Published: February 2008

A refined northeast Pacific plate‐motion model provides a framework for analysis of the Tertiary volcanic and tectonic history of western Oregon and Washington. We examine three possible models for the origin of the allochthonous Paleocene and Eocene oceanic basalt basement of the Coast Range: (1) accretion to the continent of hot spot generated linear seamount chains; (2) accretion of thick oceanic crust and seamounts generated during Farallon‐Kula spreading reorganizations between 61 and 48 Ma; and (3) eruption of basalt during oblique rifting of the continental margin as it overrode an active Yellowstone hot spot on the Kula‐Farallon ridge. The plate model suggests that microplate rotation and accretion of hot spot generated linear aseismic ridges cannot be easily reconciled with rapid northeast motion of the KuIa and Farallon plates and the well‐established paleomagnetic rotations. Following emplacement of the Coast Range basement, changes in the character of forearc, back arc and Cascade arc volcanism correlate with a marked decrease in the rate of Farallon‐North America convergence between 43 and and 28 Ma. This slowdown may be responsible for (1) westward stepping of the volcanic arc front from the Challis axis to a Cascade axis at about 42 Ma; (2) a subsequent episode of increased ash flow tuff volcanism and extension in the Cascade arc between 37 and 18 Ma that correlates with the “ignimbrite flare‐up” in the Basin and Range; and (3) a period of extensional basaltic and alkalic volcanism and intrusion in the Coast Range between 44 and 28 Ma. Reduction of the convergence rate and westward stepping of the flexure in the subducted slab may have reduced the horizontal compressive stress on the continent, allowing increased injection of magma into the crust, development of large, shallow magma chambers, and the outbreak of extensional volcanism over a large area behind the Farallon‐North America subduction zone.
The Hidaka Shear Zone (Hokkaido, Japan): Genesis during a right‐lateral strike‐slip movement
Authors: Jolivet Laurent , Miyashita Sumio
Published: February 2008

The Mesozoic structure of Hokkaido, which is characterized by a wide spread ophiolite sequence unconformably overlain by a Cretaceous flysch sequence, is cut by a north‐south trending metamorphic belt, the Hidaka Belt. This contribution aims to demonstrate that the Hidaka Belt is a wide right‐lateral strike‐slip shear zone. The movement is contemporaneous with the metamorphism and the magmatism of Oligo‐Miocene time.
Tectonic configuration of the Western Arabian Continental Margin, southern Red Sea
Authors: Bohannon Robert G.
Published: February 2008

The young continental margin of the western Arabian Peninsula is uplifted 3.5 to 4 km and is well exposed. Rift‐related extensional deformation is confined to a zone 150 km wide inland of the present coastline at 17 to 18° N and its intensity increases gradually from east to west. Extension is negligible near the crest of the Arabian escarpment, but it reaches a value of 8 to 10% in the western Asir, a highly dissected mountainous region west of the escarpment. There is an abrupt increase in extensional deformation in the foothills and pediment west of the Asir (about 40 km inland of the shoreline) where rocks in the upper plate of a system of low‐angle normal faults with west dips are extended by 60 to 110%. The faults were active 23 to 29 Ma ago and the uplift occurred after 25 Ma ago. Tertiary mafic dike swarms and plutons of gabbro and granophyre 20 to 23 Ma old are concentrated in the foothills and pediment as well. The chemistry of the dikes suggests (1) fractionation at 10 to 20 kbar, (2) a rapid rise through the upper mantle and lower crust, and (3) differentiation and cooling at 1 Atm to 5 kbar. Structural relations between dikes, faults and dipping beds indicate that the mechanical extension and intrusional expansion were partly coeval, but that most of the extension preceded the expansion. A tectonic reconstruction of pre‐Red Sea Afro/Arabia suggests that the early rift was narrow with intense extension confined to an axial belt 20 to 40 km wide. Steep Moho slopes probably developed during rift formation as indicated by published gravity data, two published seismic interpretations and the surface geology.
Geologic framework, tectonic evolution, and displacement history of the Alexander Terrane
Authors: Gehrels George E. , Saleeby Jason B.
Published: February 2008

The Alexander terrane consists of upper Proterozoic(?)‐Cambrian through Middle(?) Jurassic rocks that underlie much of southeastern (SE) Alaska and parts of eastern Alaska, western British Columbia, and southwestern Yukon Territory. A variety of geologic, paleomagnetic, and paleontologic evidence indicates that these rocks have been displaced considerable distances from their sites of origin and were not accreted to western North America until Late Cretaceous‐early Tertiary time. Our geologic and U‐Pb geochronologic studies in southern SE Alaska and the work of others to the north indicate that the terrane evolved through three distinct tectonic phases. During the initial phase, from late Proterozoic(?)‐Cambrian through Early Devonian time, the terrane probably evolved along a convergent plate margin. Arc‐type(?) volcanism and plutonism occurred during late Proterozoic(?)‐Cambrian and Ordovician‐Early Silurian time, with orogenic events during the Middle Cambrian‐Early Ordovician (Wales orogeny) and the middle Silurian‐earliest Devonian (Klakas orogeny). The second phase is marked by Middle Devonian through Lower Permian strata which accumulated in tectonically stable marine environments. Devonian and Lower Permian volcanic rocks and upper Pennsylvanian‐Lower Permian syenitic to dioritic intrusive bodies occur locally but do not appear to represent major magmatic systems. The third phase is marked by Triassic volcanic and sedimentary rocks which are interpreted to have formed in a rift environment. Previous syntheses of the displacement history of the terrane emphasized apparent similarities with rocks in the Sierra‐Klamath region and suggested that the Alexander terrane evolved in proximity to the California continental margin during Paleozoic time. Our studies indicate, however, that the geologic record of the Alexander terrane is quite different from that in the Sierra‐Klamath region, and we conclude that the two regions were not closely associated during Paleozoic time. The available geologic, paleomagnetic, and paleontologic data are more consistent with a scenario involving (1) early Paleozoic origin and evolution of the Alexander terrane along the paleo‐Pacific margin of Gondwana, (2) rifting from this margin during Devonian time, (3) late Paleozoic migration across the paleo‐Pacific basin in low southerly paleolatitudes, (4) residence in proximity to the paleo‐Pacific margin of South America during latest Paleozoic(?)‐Triassic time, and (5) Late Permian(?)‐Triassic rifting followed by northward displacement along the eastern margin of the Pacific basin.
Eocene structural development of the Valhalla Complex, southeastern British Columbia
Authors: Carr Sharon D. , Parrish Randall R. , Brown Richard L.
Published: February 2008

The Valhalla complex, a Cordilleran metamorphic core complex, is a 100 km by 30 km structural culmination within the Omineca belt of southeastern British Columbia. It comprises sheets of granitic orthogneiss ranging in age from 100 to 59 Ma with intervening paragneiss of uncertain age and stratigraphic correlation. The complex is roofed by the ductile Valkyr shear zone and the ductile/brittle Slocan Lake fault zone; the upper plate comprises lower grade metasedimentary rocks intruded by middle Jurassic plutons. The Valkyr shear zone and the Slocan Lake fault zone deform 62 and 59 Ma granitic sheets in their footwalls. The easterly directed Valkyr shear zone is a 2 to 3 km thick zone of distributed ductile strain which is arched over the complex and is exposed around the periphery on the northern, western, and southern margins. The shear zone was active between 59 and 54 Ma under amphibolite facies conditions. The juxtaposition of upper and lower plates with different structural and metamorphic histories indicates that the Valkyr shear zone is a significant structure with large displacement. There is evidence to support an easterly rooting direction consistent with an extensional origin; its surface breakaway is suggested to be west of the Valhalla complex. The Slocan Lake fault zone on the eastern side of the complex is a gently (30°), easterly dipping ductile/brittle normal fault which roots to the east. It was active between 54 and approximately 45 Ma and truncates the Valkyr shear zone. Timing and structural relationships indicate that the Valkyr shear zone and the Slocan Lake fault zone are genetically related. Movement on the ductile Valkyr shear zone, arching of the complex, and displacement on the Slocan Lake fault zone occurred as a continuum in Early to Middle Eocene time. This paper documents the presence of significant Eocene ductile strain in the Valhalla complex and suggests that the role of extension in this region is more profound than had been previously recognized.
Is extension in Death Valley accommodated by thinning of the mantle lithosphere beneath the Sierra Nevada, California?
Authors: Jones Craig H.
Published: February 2008

While relatively rapid and large‐scale extension occurred in the Death Valley region of California during Neogene time, little or no shallow extension occurred in the adjacent Sierra Nevada. This contrast in tectonic history has often been extrapolated to include the entire lithosphere, but geophysical and geologic observations indicate that more extension of the mantle lithosphere has occurred under the Sierra than under the Death Valley region. Upper mantle seismic velocities observed beneath the High Sierra are lower than those observed in other regions with comparable surface heat flow. This discrepancy could be resolved if the mantle lithosphere beneath the High Sierra had become warmer, presumably by tectonic thinning, in the last 10 m.y. Upper mantle seismic velocities, averaged topography, and Bouguer gravity anomalies all are consistent with the presence of thinner mantle lithosphere beneath the High Sierra than beneath the California portion of the Basin and Range Province to the east. This suggests that extension of the crust near Death Valley might be accommodated at a deeper level by thinning of the mantle lithosphere beneath the Sierra Nevada.

The extension in the crust of the California Basin and Range Province and the thinning of the mantle lithosphere under the High Sierra appear to share the same bounds in time and space. The uplift of the High Sierra occurred over the past 9 m.y., which coincides with most of the extension that occurred in the California Basin and Range Province. Because the orientation of extension in the California Basin and Range Province is inferred to be approximately N60°W from geologic, geodetic, and in situ stress measurements, the northern and southern edges of the Death Valley extensional subprovince may extend N60°W from the inferred northern and southern limits of west dipping low‐angle normal faults of the Death Valley region. Pronounced changes in the averaged topography and Bouguer gravity anomaly across these two bounds both in the Basin and Range Province and in the Sierra Nevada support a connection between the tectonics of both regions. The geomorphic history of the southern Sierra suggests an up‐to‐the‐north warp of the Sierra across this southern bound during latest Cenozoic time. Hence extension near Death Valley may be localized in the crust and may be laterally connected to thinning of the mantle lithosphere beneath the Sierra Nevada. This geometry requires.
Constraints on the structure of Brooks Range and Colville Basin, northern Alaska, from flexure and gravity analysis
Authors: Nunn Jeffrey A. , Czerniak Matthew , Pilger Rex H.
Published: February 2008

Tectonic thickening of the crust within the adjacent Brooks Range fold‐thrust mountain belt has been suggested as a probable cause for rapid subsidence of the Colville basin in the Early Cretaceous. A simple mechanical model in which the response of the lithosphere to loading was approximated by a semi‐infinite (broken) elastic plate overlying a weak fluid was used to analyze deflection of the basement beneath the Colville basin. Comparison between the calculated and observed depth to basement suggests that the emplacement of thrust sheets onto the lithosphere during mountain building cannot produce a load sufficient to create the observed basin geometry and observed topography regardless of the position of the slab end or flexural rigidity assumed for the lithosphere. An additional subsurface load capable of producing a deflection of the foreland basin roughly equal to the depression caused by the topographic load of the Brooks Range is required. Bouguer gravity data from the region are characterized by an asymmetric gravity low which spans the Brooks Range and Colville basin and a gravity high to the south. Gravity modeling is consistent with a subsurface load with a mass of approximately 2.5 × 1013 g/cm located beneath the southern Brooks and the Yukon‐Koyukuk Province to the south. The origin of the subsurface load is interpreted as obduction of crustal blocks onto an old rifted continental margin during attempted continental (A‐type) subduction. The Colville basin was also influenced by the contemporaneous development of the northern Arctic continental margin. Loading by subsurface and topographic loads to the south and loading on the continental margin to the north resulted in the formation of the Barrow arch just offshore of northern Alaska. A best fitting value for mechanical thickness of the lithosphere beneath the Colville basin is approximately 65 km.
EPeirogenic motions of Africa as inferred from Cretaceous shoreline deposits
Authors: Sahagian Dork
Published: February 2008

Epeirogenic motions can be quantified if paleodatum planes can be recognized. These motions have been determined for the time interval mid‐Cretaceous (Cenomanian) to recent in continental Africa using the vertical displacement of ancient shoreline sediments and other nearshore or low‐elevation deposits that are inferred to represent a Cenomanian sea level datum plane. The Cretaceous Trans‐Saharan seaway brought shoreline sediments deep into the African continental interior, and additional data suggest that most of the remaining terrestrial regions were also low‐lying during the Cenomanian. The present elevations of the sea level sediments indicate that large portions of Africa have undergone epeirogenic uplift since the Cenomanian. The character of the motions in north Africa can be described as small‐scale high‐amplitude (3 km) doming separated by relatively flat‐lying areas of deposition. The largest single epeirogenic feature is the uplift associated with Red Sea and East African Rift development. The timing of post‐Cenomanian epeirogenic motions can be further constrained using paleontologic, K‐Ar, faulting, paleodrainage and sedimentary data. The most likely causal mechanism for African epeirogeny may be hot spot activity reflective of the mantle convection regime underlying the African lithosphere.
Continental erosion and large‐scale relief
Authors: Pinet Patrick , Souriau Marc
Published: February 2008

A worldwide investigation of continental erosion is carried out by the study of large drainage basins, on the basis of hydrological data, environmental factors, and basin relief distribution. Inside each basin, mean geochemical and mechanical denudation rates are defined. A multicorrelation analysis shows that the mechanical denudation rates Ds are uncorrelated with environmental factors and correlated with mean basin elevation H, while chemical denudation rates Dd are insensitive to relief but correlated with mean annual precipitation. Furthermore, two linear relationships between H and Ds are detected: (1) Ds (m/10³ yr) = 419×10−6 H (m) ‐ 0.245, with V (explained variance) = 95.1%; this law concerns basins related to orogenies younger than 250 Ma. The negative intercept is interpreted as a continental sedimentation rate of 245 m/m.y. An alternative model in which one invokes a critical elevation, separating erosion from sedimentation, is equally successful and leads to lower sedimentation rates (60–110 m/m.y.). For both models, one derives from the slope of the adjustments, erosion time constants on the order of 2.5 m.y. (2) Ds (m/10³ yr) = 61×10−6 H (m), with V = 86.5%; this law concerns basins related to older orogenies. The null intercept suggests the lack of continental storage. Because of the more important dispersion of the data, the erosion time constant is calculated separately for each basin; it ranges from 15 to 360 m.y. The tectonic implications of these results are discussed. In particular, the short time constant 2.5 m.y. agrees with orogenic uplift rates on the order of 1 mm/yr, observed in active mountain chains.
Paleomagnetism of Tertiary rocks from the OGA Peninsula and the rotation of northeast Japan
Authors: Tosha Toshiyuki , Hamano Yozo
Published: February 2008

We have made a paleomagnetic study of Tertiary rocks from the Oga Peninsula, northern Honshu Island. Volcanic and sedimentary rocks were sampled from 26 sites in the peninsula. The ages of the rocks range from 62 Ma to the present. The results indicate a counterclockwise rotation of Northeast Japan with respect to eastern Asia around a pole at 58°N, 149°E, between about 22 Ma and 15 Ma. The amount of the rotation is about 20°. Before the rotation, Northeast Japan was situated along the east coast of the Asian continent.
Possible paleostress tensor configurations derived from fault‐slip data in eastern Vermont and western New Hampshire
Authors: Hardcastle Kenneth C.
Published: February 2008

The configuration of six possible paleostress tensors have been derived from 152 faults measured in eastern Vermont and western New Hampshire. Populations of potentially genetically related faults were separated using two techniques. Tensor configurations for each population were derived using a linear least squares inversion method based in part on the work of Reches [1987] and a grid search inversion method which tests over 100,000 possible tensors for compatibility with all or portions of the data. Faults belonging to the oldest population (set R; n=24) occur primarily in high‐grade rocks. This set is composed of semiductile, northeast trending reverse and west–northwest trending left‐lateral faults. Fault fabrics of quartz rods and thin mylonite layers suggest that the host rocks were at crustal levels of 8–10 km during faulting. The derived tensor indicates a roughly east–west σ1 and near vertical σ3. Set R faults are offset by normal faults (sets N1 and N2) and are interpreted to be pre‐Mesozoic in age, perhaps related to late Paleozoic Alleghanian compression. Normal and normal oblique faults mineralized with chlorite, calcite, and strained quartz (n=73) have been separated into two populations (sets N1, n=49; and N2, n=24) even though these faults are likely of similar age. Host rocks were probably at moderate crustal depths of perhaps 5 km during faulting. Faults of both sets are most likely related to Mesozoic rifting. Tensor configurations indicate that σ1 plunges steeply northwest (N1) and southeast (N2), and σ3 plunges gently, roughly east–west. These faults cut presumably Mesozoic age dikes and are themselves offset by normal oblique faults (set T) and right‐lateral faults (set RL). Strongly deviatoric, near vertical σ1 suggests a thermally driven tectonic regime during the development of normal faulting in New England. Set T (n=25) is composed of normal oblique slip faults mineralized similarly as sets N1 and N2. The plunge of σ1 is 50°N and σ3 plunges gently east. Field relations indicate set T faults document the transition from normal (sets N1 and N2) to strike‐slip faulting regimes (set RL). A compatible sequence of Mesozoic stress fields is suggested de Boer et al. [1988]. North–northeast trending, right‐lateral faults of set RL (n=7) are mineralized with quartz‐limonite‐calcite‐chlorite. Calcite twins are bent and fractured, and quartz grains show minor internal strain, suggesting that these faults developed while host rocks were at relatively shallow crustal levels. The plunge of σ1 is gently northeast and, σ3 plunges gently east–southeast. Seven distinctly mineralized (feldspar‐siderite‐quartzoalcite‐1imonite‐chlorite) gently northwest dipping, thrust faults of set F define the sixth tensor. These faults are cut by essentially vertical, east‐side‐up faults and are assumed to be Mesozoic in age by analogy with similar faults in western Massachusetts.
Late Cenozoic Basins of northern California
Authors: Nilsen Tor H. , Clarke Samuel H.
Published: February 2008

The late Cenozoic basins of northern California developed in response to both convergent tectonics associated with subduction of the Farallon plate (and its modern representatives, the Juan de Fuca and Gorda plates) and transform tectonics associated with northward migration of the Mendocino triple junction and formation of the San Andreas fault system. The modern Eel River basin north of the Mendocino triple junction is an active forearc basin located between the Cascade magmatic arc to the east and the trench at the foot of the continental slope of northern California and Oregon to the west. Five different types of late Cenozoic basins or fragments of basins are preserved in onshore northern California south of the Mendocino triple junction: (1) remnants of a formerly more extensive Neogene forearc basin preserved locally in downdropped blocks within the San Andreas fault system; (2) remnants of slightly older trench‐slope basins that generally developed west of but which may locally structurally underlie the younger forearc basin; (3) younger strike‐slip‐related structural basins that have developed along active right‐lateral faults of the San Andreas fault system; (4) broad shallow embayments, perhaps similar to the modern San Francisco Bay, that were connected to the deeper Pacific Ocean to the west; and (5) structurally emplaced remnants of oceanic crust and its overlying sedimentary cover (the King Range terrane). Our preliminary stratigraphic and sedimentologic studies suggest that much of northern California was covered during the Neogene by a forearc basin that may have extended as far south as the San Francisco Bay region and into the southern San Joaquin Valley. As the Mendocino triple junction migrated northward during the late Cenozoic, the southern margin of the forearc basin was uplifted, basin deposits were stripped off by erosion, and the locus of forearc sedimentation shifted progressively northward through time. Preserved but isolated fragments of the forearc basin provide a partial record of the original paleogeography and tectonic framework of northern California during the late Cenozoic.
Stress and the direction of slip on fault planes
Authors: Gephart John W.
Published: February 2008

The shear stress direction on a fault plane depends only on four of the six components of the stress tensor. Assuming only that the slip direction marks the shear stress direction on any fault plane (and that stress is homogeneous), it is possible to estimate these four stress parameters from populations of fault planes with known slip directions, as several workers have observed. Different formulations of the problem may yield varying best‐fitting stresses and estimates of uncertainty. In the simplest case, no assumptions are made regarding the orientations of fault planes relative to the stress tensor; thus the technique allows for the possibility that the fault planes may be very weak. Here we present simple analytical and graphical descriptions of the field of admissible fault geometries relative to any four‐parameter stress model, which can be used to illustrate the significance of various inverse strategies. In particular, this paper explores the effects of using two alternative measures of misfit between an observed fault datum and stress model: (1) the pole rotation (the angle between the observed and predicted slip direction on the observed fault plane), and (2) the minimum rotation (the smallest angle between the observed fault geometry and any fault geometry which is consistent with the model). By allowing for variation of the fault plane as well as the slip vector, the minimum rotation procedure generally achieves a more stable and (presumably) realistic estimate of the actual discrepancy between a fault observation and stress model than the pole rotation procedure. In a test case using 17 earthquake focal mechanisms from the YuIi region of eastern Taiwan, separate inversions based on the two misfit criteria yield different optimum stress models and uncertainty estimates. Additional constraints on the stress tensor, such as the effect of friction, can be superimposed on the ones used here.
Structural evolution of the Early Tertiary Cantwell Basin, south central Alaska
Authors: Hickman Robert G. , Sherwood Kirk W. , Craddock Campbell
Published: February 2008

The Cantwell Basin is a Paleogene intermontane basin located in south central Alaska. The Cantwell Formation which fills the basin is as much as 4000 m thick and consists of a lower sedimentary sequence of fluvial and alluvial fan origin and an upper sequence of calc‐alkaline volcanic rocks. The east trending Cantwell Basin developed as a southward thickening asymmetrical graben between the Hines Creek and Denali faults. Substantial dip‐slip displacement and local, dextral strike‐slip displacement on the Hines Creek fault during basin subsidence was followed by dip‐slip displacement in late Cenozoic time. Strike‐slip movement along the Denali fault, to the south, exerted primary control on basin subsidence and subsequent structural inversion. During the Paleocene, Kula plate motion was nearly perpendicular to the axis of the Cantwell Basin; however, Kula plate motion was oblique to southeastern Alaska and produced regional dextral slip on the Denali fault system. The Cantwell Basin appears to have developed as a pull‐apart basin in which northwest striking syndepositional normal faults linked the Denali fault with the western part of the Hines Creek fault. In early Eocene time an increased rate of plate convergence resulted in substantial displacement on the Denali fault. Concurrent counterclockwise rotation of southern Alaska produced a regional constraining bend in the Denali fault and resulted in strong compressional deformation of the Cantwell Basin.