Clockwise Sense Research Articles

Palaeomagnetism and palaeogeography of Variscan formations of the Bohemian massif: A comparison with other regions in Europe

Statistical evaluation of palaeomagnetic data from the Early Carboniferous to the Middle Triassic rocks in Europe, north of the Alpine tectonic belt, confirmed previously defined palaeotectonic stability of the whole European Plate since the Early Permian. The Trans-European Suture Zone represents a plate boundary, SW of which the Early Variscan and pre-Variscan formations show different degrees of palaeotectonic rotations, predominantly rotations of clockwise sense. A theoretical model simulating the translation and rotation movements was proposed showing that the West European Variscides underwent Hercynian palaeotectonic rotations comparable with the rotations derived for the Alpine tectonic belt.

Paleomagnetic and structural evidence for localized tectonic rotation associated with fault drag in the northeastern Mojave Desert: Implications for the late Cenozoic tectonic evolution of the Eastern California shear zone

Paleomagnetic data, coupled with detailed geological analysis of the southeastern Goldstone Lake region, indicate that lower Miocene volcanic and epiclastic rocks of the Pink Canyon area have been folded, faulted and tectonically rotated ∼ 28.4 ± 9.0° clockwise about a vertical axis; identical rocks lying to the west across the Goldstone Lake Fault (east branch) have been rotated 9.6 ± 7.4°. The Coyote Canyon Fault is locally folded about a vertical axis ∼ 25° in a clockwise sense in the Pink Canyon area. Timing relationships indicate that rotation is post-early Miocene; regional relationships imply that deformation is late Miocene to Holocene in age. These relationships imply that tectonic rotation is local rather than regional in extent as proposed by some tectonic models. The results of this study are generally consistent with the Dokka and Travis [1] model and the subsequently revised Dokka model [2] of strain partitioning in the northeastern Mojave Desert block.

Shear zones formed along long, straight traces of fault zones during the 28 June 1992 Landers, California, earthquake

Abstract Surface rupturing during the 28 June 1992 Landers, California, earthquake, east of Los Angeles, accommodated right-lateral offsets up to about 6 m along segments of distinct, en-echelon fault zones with a total length of 80 km. The offsets were accommodated generally not by faults—distinct slip surfaces—but rather by shear zones, tabular bands of localized shearing. Along simple stretches of fault zones at Landers the rupture is characterized by telescoping of shear zones and intensification of shearing: broad shear zones of mild shearing, containing narrow shear zones of more intense shearing, containing even narrower shear zones of very intense shearing, which may contain a fault. Thus the ground ruptured across broad belts of shearing with clearly defined, subparallel walls, oriented NW. Each broad belt consists of a broad zone of mild shearing, extending across its entire width (50 to 200 m), and much narrower (a few meters wide) shear zones that accommodate most of the offset of the belt and are portrayed by en-echelon tension cracks. In response to right-lateral shearing, the slices of ground bounded by the tension cracks rotated in a clockwise sense, producing left-lateral shearing, and the slices were forced against the walls of the shear zone, producing thrusting. Even narrower shear zones formed within the narrow shear zones. Although these probably are guides to right-lateral fault segments below, the surface rupturing during the earthquake is characterized not by faulting, but by the formation of shear zones at various scales.

Polarisation Characteristics of Pulsations Associated with Storm Sudden Commencements.

Polarisation properties of magnetic pulsations associated with SCs recorded at an equatorial station, Choutuppal (Gm. Lat.: 7.5°N), during the years 1970-88 have been analysed. The results show that in contrast to a well defined local time dependent sense of rotation of polarisation vector reported for high and middle latitude pulsations (Wilson and Sugiura, 1961; Lanzerotti et al., 1981), the equatorial pulsations exhibit a predominantly clockwise sense of rotation particularly from around 22 hrs to early afternoon hours (13 to 14 hrs LT) while the events occurring later in the afternoon exhibit, in addition, a counter clockwise sense as well. The results for this equatorial station are consistent with the observations reported for very low latitude stations (Yumoto, 1986), thus indicating a latitudinal factor in characterising the local time dependence of sense of rotation of polarisation vector. The results are discussed in relation to the existing models invoking the azimuthally travelling ionospheric eddy currents for the immediate source of equatorial pulsations.

East-west thrusting and anomalous magnetic declinations in the Sierra Gorda, Betic Cordillera, southern Spain

Structural and palaeomagnetic studies in the Sierra Gorda (Sierra de Loja), located in the External zone of the Betic Cordillera, indicate that westward-directed thrusting is not associated with significant rotations about a vertical axis. Detailed mapping and slip vector analysis show that the Sierra Gorda is a thrust complex composed of three thrust sheets. The uppermost thrust places Early Jurassic pelagic carbonates on top of Jurassic to Oligocene sediments that form a large doubly-plunging footwall syncline. The eastern limb of this syncline has been overturned and is tectonically thinned as a result of the overthrusting.Palaeomagnetic results from Mesozoic and Tertiary sediments both within and around the perimeter of the Sierra Gorda indicate that: (1) the average remanence vector of the seven Late Jurassic localities sampled within the Sierra Gorda has a direction (D = 328° and I = 38°) that is not significantly different from the expected declination for the Upper Jurassic of stable Iberia; and (2) there is no significant difference between the remanences in the two upper thrust sheets indicating that differential rotation did not occur during the initiation and displacement on the thrusts. In contrast, the one Late Jurassic site that was sampled to the west of the Sierra Gorda is rotated, like the rest of the Subbetics, 60° clockwise of the reference direction. The unrotated directions obtained in the Sierra suggest, either that it has rotated in a clockwise sense concordant with the rest of the Subbetic zone and has then been backrotated, or that it has never rotated relative to stable Iberia. In the latter, simpler hypothesis the unrotated declinations may be explained in terms of orthogonal convergence along an irregular continental margin.

A two‐dimensional array study of low‐latitude PC 5 geomagnetic pulsations

Data obtained from the Australia Wide Array of Geomagnetic Stations (AWAGS) have provided a unique opportunity to investigate the spatial variation of long‐period (Pc 5) ULF wave signal characteristics as a result of plasma waves in low latitudes. Pure state filtering techniques were applied in order to detect highly polarized Pc5 events and derive the wave signal parameters, i.e., the wave frequency, amplitude, polarization, and phase. The results indicate that the Pc 5 signal frequency was virtually independent of latitude and longitude, while the amplitude decreased considerably toward lower latitudes. The spatial variation of the signal polarization was found to be small. However, a statistical analysis of the signal polarization revealed a systematic diurnal variation. Counterclockwise sense of rotation (viewed in the direction of the ambient magnetic field) with the major axis of the polarization ellipse in the northeast quadrant was observed in the local morning, while clockwise sense of rotation with the polarization major axis aligned mainly in the north‐south direction was seen in the local afternoon. Interstation phase analysis yielded small phase variations across the entire station array, indicating azimuthal wave numbers of 1<m<3. The signals appeared to propagate westward in the local morning sector, while eastward propagation prevailed in the local afternoon. The latitudinal phase variation was also very small, showing signals virtually in phase or with the lower‐latitude stations leading slightly. These observations appear to be consistent with the magnetic ground signature of global compressional modes or large scale‐cavity resonances trapped in the magnetosphere.

Rifting of Africa and pattern of mantle convection beneath the African plate

Abstract The Mesozoic and Cenozoic tectonic setting of the African continent is characterized by an extensional stress regime, as is evident from the evolution of the West, Central and East African rift systems, the opening of the Indian and Atlantic oceans and the development of Tethys domain, as well as from the results of geophysical investigations. The most prominent feature of the Bouguer gravity anomaly map of Africa is a great belt of negative Bouguer anomalies which extends across the entire African continent from Ethiopia in the East to Benguela in the West. In conjunction with the Red Sea spreading zone this belt appears to encircle a “centre” in Cameroon. It probably corresponds to a distinct zone of lithospheric thinning and extension. The entire African continent is characterized by higher-than-normal elevations. It is located inside the African residual geoid high. Earthquake fault-plane solutions confirm the extensional character of the present tectonic regime of eastern and southern Africa where normal faulting predominates. The average orientation of T axes is N-S in South Africa, NW-SE in Zambia, Mozambique and Madagascar, E-W in Kenya and Ethiopia, and NE-SW in the Red Sea region. From north to south, the orientation of T axes rotates systematically in a clockwise sense by 130° and describes a radial pattern centred on equatorial West Africa. This indicates that the African lithosphere has been subject to continent-wide radial extension around a hypothetical West African “spreading centre”, referred to as the African Spreading Centre A (ASC-A), which coincides with the geographical centre of the African plate. Shear traction exerted by the convecting mantle on the base of the lithosphere is considered to be a major driving force of plate movements. In the case of Africa, the continent-wide regularities in the pattern of lithospheric extension, the elevated position of the continent, as well as the evolution of the mid-oceanic ridge system surrounding Africa, provide distinct boundary conditions for the pattern of large-scale mantle flow beneath the African plate. It is proposed that relatively warm and less dense mantle material rises below Africa, forming a single mega-plume or a number of plumes. At the base of the African lithosphere, the ascending material diverges and flows radially away from the centre of ascent, corresponding to the ASC-A. This ascending flow forms part of a very large mantle convection cell which occupies the Eurafrican hemisphere. The descending branch of this convection cell is located at a distance of 80–90° from the ASC-A. The mid-oceanic ridges surrounding the African continent are located between the upwelling and the downwelling branches of this convection cell, at a distance of 50–60° from the ASC-A. Upper mantle flow below the sea-floor spreading axes is uni-directional and horizontal and is directed away from the ASC-A.

Integration of east African Paleostress and present‐day stress data: Implications for continental stress field dynamics

Borehole breakout studies, aligned Quaternary volcanic vents, and kinematic analysis of Quaternary faults indicate that the present‐day least horizontal stress direction (Shmin) in Kenya is aligned approximately NW‐SE and in central Sudan is nearly N–S. Limited data in eastern Ethiopia suggest a NE‐SW Shmin orientation. The regional pattern for Shmin is therefore roughly radially disposed about the Afar plate junction. Geologic structures in the Gregory Rift, western Kenya, suggest that up to 0.6–0.4 Ma, Shmin was oriented E‐W, normal to the main rift trend. Similar arguments for the Ethiopian Rift indicate that during its main phases of extension, Shmin was oriented NW‐SE. These data indicate that the central east African stress field underwent a significant realignment during the Quaternary, rotating about 45° in a clockwise sense in the Kenyan Rift. Similar rotations may have occurred in eastern Sudan and eastern Ethiopia on the periphery of the Ethiopian Rift. In Kenya, this stress reorientation resulted in dextral oblique reactivation of older normal faults and formation of new normal faults with dextrally oblique slip components. The large‐scale structural framework of the Kenyan rift is no longer suitably oriented to accommodate large extensional strains. East African extension may now be transferred to the western branch of the rift system (Tanganyika‐Malawi rifts) and rejuvenated basins south of the Gregory Rift in Tanzania (Eyasi‐Manyara rifts). Rapid rotations of near‐surface stress tensors (of the order of 7.5°/105 years in the case of East Africa) are now documented in several continental extensional systems. It is unlikely that large‐scale mantle circulation patterns can fluctuate at such rapid rates. This suggests that drag at the base of the lithosphere, at least in continental extensional areas, is not the dominant force controlling the orientation of the near‐surface stress field. Rather, intraplate forces, produced and propagated from distant plate boundaries (ridge‐push, subducting‐slab‐pull), may play a greater role in configuring the continental crustal stress regime.

Progressive deformation and the rotation of contemporary fold axes in the Ballybofey Nappe, north‐west Ireland

AbstractInternal regions of orogenic belts may be characterized by an alignment of fold axes with mineral elongation lineations. This relationship is commonly interpreted as representing progressive tightening and rotation towards the shear direction of early buckle folds, the hinges of which were initiated orthogonal to this direction. Detailed structural analysis of lower amphibolite facies Dalradian metasediments of the Ballybofey (fold) Nappe, north‐west Ireland, shows that an intense S3 schistosity is developed axial planar to mesoscopic and minor F3 folds. In areas of low D3 strain, F3 fold axes plunge gently towards the north‐east, whereas in regions of greater strain plunges are towards the south‐east subparallel to the constant mineral lineation. Minor folds which initiated at angles of 70–80° from the mineral lineation subsequently rotated towards the shear direction in a consistent clockwise sense. Progressive and variable non‐coaxial deformation oblique to the original mean F3 orientation has resulted in a unimodal distribution pattern of fold axes. Analysis of the angular rotation of fold axes enables estimates of the bulk shear strain to be evaluated and models of progressive deformation to be assessed.

Paleozoic Structure of the Central Basin Uplift and Adjacent Delaware Basin, West Texas (1)

Structural cross sections and a regional tectonic map show that the Central Basin uplift consists of upthrust blocks of alternating vergence. The uplift is separated from the adjacent Delaware basin by a major fault that changes throw and structural style along strike, accompanying changes in block vergence found along the uplift. The west-facing basement blocks were thrust westward over the Delaware basin, whereas the adjacent blocks face eastward. The distribution and style of structures suggest that small amounts of left-lateral movement occurred along west-trending and northwest-trending faults that separated rising blocks of the uplift. Transfer of lateral movement from vertical, west-trending faults on the Central Basin uplift and the Ozona arch to the west-trending Mid-basin fault in the Delaware basin rotated blocks of the uplift in a clockwise sense. Faults and folds within the study area have trends and slip directions that are generally similar to those within the Reagan and Washita Valley fault zones in the Arbuckle uplift adjacent to the Anadarko basin. The difference in the style of deformation between the two uplifts relates to varied angular relationships between preexisting structure and the direction of late Paleozoic stress. The identity of structural trends within these two uplifts suggests that other late Paleozoic basins and uplifts of the Ancestral Rocky Mountains may have formed in a similar stress field and that crustal rotation was pervasive along the southwestern margin of the continent during the late Paleozoic Ouachita-Maratho deformation.

Siluro-Devonian palaeomagnetism, terrane emplacement and rotation in the Caledonides of western Ireland

SUMMARY The palaeomagnetism of a Silurian succession (Lough Mask Formation lavas and sediments) and intrusive rocks (Kilbride dolerites and andesites and microgranodiorites, Killary Harbour area) in the Connemara-Mayo segment of the Irish Caledonides is described. Magnetizations are related to the regional tectonic history with the aid of fold, conglomerate and contact tests. They describe sequences of shallow to intermediate inclinations in both the eastern and western sectors of this Caledonian inlier. The shallow components appear to pre-date Siluro-Devonian folding while the intermediate ones post-date this episode. The change in palaeofield inclination is identical to the transition described by contemporaneous deep level (Silurian) to shallow and surface level (Lower Devonian) bodies in the British and Scandinavian Caledonides but declinations are rotated consistently to the west. The Lower-Middle Silurian vector is rotated clockwise by c. 100. This difference is progressively reduced to c. 50 by the time of the Siluro-Devonian D2 folding, showing that this fold generation (now oriented E-W) developed in parallelism with contemporaneous folds on the NE strike continuation of the Caledonides. Post-folding remanences continue to deviate in a clockwise sense from the remainder of the Caledonides showing that rotation continued into Middle-Upper Devonian times and finally ceased during Carboniferous times. The integrated effect of this rotation is recognized in Ordovician magnetizations from the Dalradian metamorphic terrane of south Connemara. It can be explained by block rotations within a zone of distributed deformation according to the McKenzie-Jackson model and illustrated by many neovolcanic zones. Palaeomagnetism defines a tectonic regime commencing with rotations of c. 3Myr-' and lasting from mid-Silurian to Carboniferous times. Western Ireland is sited within a closure gap along the Iapetus suture extending from Ireland to Newfoundland, and these terranes were apparently emplaced by progressive elimination of this gap during sinistral transpression across the Caledonian orogen.

Paleomagnetism of three upper Jurassic ash‐flow sheets in southeastern Arizona: Implications for regional deformation

A sequence of three petrologically distinct Upper Jurassic ash‐flow tuffs has been sampled for paleomagnetic analysis in the Huachuca Mountains, Canelo Hills, and Mustang Mountains of southeastern Arizona. Site‐mean paleomagnetic directions for these units indicate significant vertical‐axis rotations between sampling localities (≃15°–40°), and a comparison of these and previously published data with reference directions from the Colorado Plateau implies that the Jurassic rocks in southeastern Arizona have been rotated in a clockwise sense with respect to stable North America. These new data are consistent with paleomagnetic results showing clockwise rotation of Upper Cretaceous rocks in south central Arizona, and further indicate that studies of North American apparent polar wander based on rocks from southern Arizona could be subject to error. The clockwise rotations in southern Arizona are likely related to Late Cretaceous and early Tertiary strike‐slip movement on northwest‐trending high‐angle faults in the region, and this movement may be linked to an increase in oblique Farallon‐North American plate convergence at that time.

Interaction of a Warm Ring with the Western Slope in the Gulf of Mexico

Abstract Between November 1985 and May 1986, a warm ring encountered the western slope in the Gulf of Mexico, moved away from the slope, and began to dissipate. Before encountering the slope, the ring was quasi-circular. After encountering the slope, it became elliptically shaped with a center of circulation that changed position rapidly and erratically. The major axis of the ring rotated rapidly in a clockwise sense as the ring increased in size, most probably through interaction with two other rings in the western Gulf. During this period, neither satellite nor in situ data indicated large-scale exchange of shelf and ring water. When the ring moved eastward, then southward, away from the slope, it was observed to decrease in size by a factor of 3 to 5; the upper layer manifestation of the ring had weakened considerably. To the north of the center of the ring was a cyclonic feature which was apparently coupled to the ring and remained so throughout the period examined. The cyclone was quasi-circular and ...

Effects of diffusional modification of garnet growth zoning on P-T path calculations

The effect of intragranular diffusion on chemical zoning in garnet and on P-T paths calculated from that zoning was evaluated using a numerical model of multicomponent diffusion in combination with simulations of garnet growth. Syn-and post-growth diffusion of Mg−Fe−Mn−Ca species in garnet was calculated for a model pelitic assemblage over a range of temperatures from 485 to 635°C. Compositions from zoned garnet, as modified by diffusion, hypothetical inclusions of plagioclase within garnet and matrix phases were used to reconstruct pressure-temperature (P-T) paths from isobaric and polybaric model histories. P-T path calculations, based on numerical simulations conducted over an input isobaric heating path that reached peak temperatures between 585 and 635°C, show that relaxation of garnet compositional gradients by diffusion can induce modest to appreciable curvature in the inferred paths. Retrieved paths also indicated somewhat smaller overall temperature changes relative to the actual temperature difference of the input path. The magnitude of these distortions is shown to depend upon the heating and cooling rate and garnet crystal size as well as the actual peak temperature condition. The effect of diffusion on path trajectories in simulations with thermal histories that also included cooling were comparable to heating-only models that reached peak temperatures approximately 15–30°C higher. Compositions of garnets with radii less than 1 mm, that reached actual peak temperatures of 605°C along temperature-time histories characteristic of regional metamorphism, experienced sufficient diffusional relaxation to introduce errors of hundreds of bars to in excess of one kilobar in path trajectories. Path distortions were significant at heating/cooling rates less than 10°C/Ma, but rapidly diminished for rates faster than this. In polybaric simulations diffusion effects were least noticeable when the actual pressure-temperature conditions changed in a clockwise sense (i.e., convex to higher P and higher T), but apprecciable modification was seen in path models that underwent counterclockwise changes in P and T. Reequilibration of garnet rim compositions occurred during cooling on all paths, and temperature maxima obtained from garnet-biotite geothermometry underestimated actual peak conditions by 40 to 70°C. Calculations suggest that P-T path trajectories calculated from garnets of at least 1 mm size, and that experienced actual thermal maxima below 585°C, are not likely to be distorted by diffusional effects during regional metamorphism. However, P-T path reconstructions based on garnet zonation with smaller grains or higher temperatures may lead to misinterpretation of crystallization history. The partitioning record of peak metamorphic temperatures may be destroyed by diffusional reequilibration of garnet rim compositions during cooling, seriously complicating the task of quantitatively estimating diffusion effects on path calculations.

Observational studies of jets in extragalactic objects. II. Three extensions

Using the equipment described in [3] we observed PKS 0349-27, 3C 98 and PKS 0634-20 on 1988 January 23–25. We found: that the protrusions on the outer edge of PKS 0349-27 all showed a tendency of inclining in the clockwise sense; that the extension in 3C 98 extended mainly in the northeast direction; and that the extension in PKS 0634-20 extended in the north-south directions, mainly to the north.

The Tectonic Mechanism for Uplift and Rotation of Crustal Blocks in the Central Basin Platform, Permian Basin, Texas and New Mexico

The Central basin platform is a positive tectonic element in the subsurface of the Permian basin. This enigmatic platform strikes north-northwest-south-southeast and at a high angle to the Marathon fold-and-thrust belt to the south. Although the uplift of the platform was related temporally to major overthrusting in the orogenic belt to the south and east, its formative mechanisms are still poorly understood. Previously compiled tectonic maps and cross sections were analyzed to identify the significant characteristics of this complicated structure. (1) Much of the platform is bounded by laterally discontinuous, high-angle faults with large vertical displacements. (2) The bounding faults suggest that the platform is composed of several discrete blocks that are arranged in a dextral en echelon pattern. (3) The southwest and northeast corners of each block typically are bounded by major faults; block uplift is greatest at the southwest and northeast corners. (4) Blocks are separated by west-northwest-east-southeast-trending transfer zones. These characteristics suggest that the Central Basin platform was subjected to a north-northwest-south-southeast-trending dextral couple that caused the platform to split into several blocks. Individual blocks rotated in the same clockwise sense and produced the maximum uplift observed at the southwest and northeast corners of blocks. In additionmore » to the above characteristics, the amount of uplift an the width of individual blocks progressively decrease toward the north; block boundaries also become less defined northward. However, these additional complexities are not fully understood yet.« less

Palaeomagnetic evidence for clockwise rotations related to dextral shear along the Southern Troodos Transform Fault, Cyprus

The Southern Troodos (Arakapas) fault represents a Late Cretaceous oceanic fracture zone which was locally disrupted during Late Cretaceous-Eocene palaeorotation of the Troodos microplate. Opinions are divided as to the sense of displacement along the transform fault and the exact timing of initiation of palaeorotation. Palaeomagnetic studies of Turonian zeolite facies lavas and sediments exposed within the transform domain have revealed considerable variations in the declination of remanent magnetisation between sites along the Arakapas fault belt, the western margin of the Limassol Forest Complex and the eastern flank of the Troodos ophiolite. At seven sites clockwise rotation of fault blocks has occurred about steeply inclined axes. One fault block at the western end of the Arakapas fault belt has experienced a net anticlockwise rotation, while at six other sites only simple tilting about sub-horizontal axes is indicated. The overall clockwise sense of block rotation and initial dyke strikes calculated at three sites are consistent with right-lateral slip along the transform. Cross-cutting relationships revealed by the analysis of one site demonstrate that these rotations took place during crustal genesis and are not a product of post-spreading disruption of the fracture zone. Sites located further south, in the Eastern Limassol Forest Complex, show no relative rotation with respect to the main Troodos ophiolite to the north and have experienced only simple tilting. However, if our results are considered in conjunction with existing palaeomagnetic data from the area, it appears that the entire area lay within a complicated zone of localised and predominantly clockwise block rotations produced by dextral slip along the transform. Some areas were rotated by over 100° about steeply inclined axes, whereas others experienced only simple tilting. The whole area was later subjected to a bulk 90° anticlockwise rotation along with the Troodos microplate. Data obtained from the umbers and radiolarites (Perapedhi Fm.) exposed on the eastern flank of the Limassol Forest block indicate that at least 30°, and possibly up to 45°, of this 90° rotation took place over a maximum of 15 Ma.

Paleomagnetism and geochronology of St. Matthew Island, Bering Sea

Oriented samples from a series of flow units, pyroclastics, tuffs, tuff breccias, and some intrusive rocks from St. Matthew Island were studied paleomagnetically in an attempt to constrain possible paleolatitudes of the Bering Sea shelf. The volcanic sequences have apparently stable magnetic directions and contain a reversed–normal–reversed–normal polarity succession.The Late Cretaceous paleolatitude deduced from the paleomagnetic data was 67°N, and the island has rotated in a clockwise sense by a nominal 10°. This implies that the rocks analyzed were formed about 12 °south of their present location with respect to cratonic North America. These data argue against tectonic models for the Bering Sea shelf region that require large-scale Late Cretaceous and Tertiary latitude changes.New K–Ar age dates combined with previously reported K–Ar ages give a best estimate of the age of the St. Matthew volcanic units of about 78.7 ± 0.4 Ma. The reversed–normal–reversed polarity changes are thought to represent part of the sequence seen between marine anomalies 32 and 33 that are usually assigned an age of about 74 Ma, which appears to be about 5 Ma too young.

Orientation of the CN X 2Σ + fragment following photolysis of ICN by circularly polarized light

A low-pressure vapor of cyanogen iodide (ICN) is photolyzed by a beam of circularly polarized radiation at 248 nm from a KrF excimer laser. The resulting ground state CN X 2Σ + fragments are probed by laser-induced fluorescence (LIF) via the CN B 2Σ +-X 2Σ + band system. The probe beam is directed antiparallel to the photolysis beam and is alternately left-circularly or right-circularly polarized. By analyzing the variation of the LIF signal with respect to the circular polarization of the probe light, it is deduced that the CN photofragments are oriented, that is, the CN molecules rotate in space with a preferred clockwise or counterclockwise sense. The degree of orientation varies with individual rotational levels for the CN(ν=0) fragments, and the orientation even changes direction as a function of the CN rotational quantum number N. Strong evidence is presented indicating that the CN orientation direction correlates with the 2P 3 2 or 2P 1 2 fine-structure level of the iodine atom half-collision partner. The CN X 2Σ +(ν=2) fragments are formed only with ground state ( 2P 3 2 ) iodine atoms because of energetic constraints. For these photofragments, the F 1 ( J = N − 1 2 ) and F 2 ( J = N + 1 2 ) fine-structure components appear to be nearly equally and oppositely oriented so that the net orientation vanishes unless the individual spin-rotation fine-structure components are resolved. It is proposed that spin-orbit interaction causes the photofragment orientation, in analogy to how this interaction produces spin-polarized electrons in the photoionization of atoms or molecules by circularly polarized light.

Three-dimensional model of the human craniofacial skeleton: method and preliminary results using finite element analysis

The purpose of this study was to develop a three-dimensional finite element model of the craniofacial skeleton using a dry human skull. The model consisted of 2918 nodes and 1776 solid elements, and was used to investigate the biomechanical effect of a distally directed orthopaedic force on the craniofacial complex. The force was applied at the level of the maxillary first molar. The results indicated that in response to the force system applied: the nasomaxillary complex displaces in a backward and downward direction and rotates in clockwise sense; the nasomaxillary complex, including the zygomatic bone, experiences high stress levels in comparison with those at the remaining bones; the stress distribution in the maxillary basal bone area is relatively uniform; and the stress distribution across the opposing surface of the bony margins of the sutures is non-uniform.