Rotational History Research Articles

Management and Crop Residue Influence Soil Aggregate Stability

AbstractSoil management is one of the most important factors influencing the structure of soils. The interaction of management (including tillage and crop rotation history) with soil biochemistry, soil aggregation, and soil humus composition was determined in a native prairie and a producer field situation in 1997. A comparison of a native prairie and an adjacent conventional corn (Zea mays L.)‐soybean [Glycine max (L.) Merr.] rotation on the same soil type found that the Webster soil (fine‐loamy, mixed, superactive, mesic Typic Endoaquoll) after soybean (C3 plant) was lower in monosaccharide content and protein content as determined by ion chromatography, and lower in phenolic acid content than the Webster soil after corn (C4 plant) or in native prairie as determined by gas chromatography. A wet, nested sieve aggregate stability measurement determined that the prairie soil had a higher mean aggregate size (1.85 mm) when compared with the soil in the presence of decomposing corn (1.0 mm) or soybean (0.34 mm) residues. Mean aggregate size was found to be correlated with soil monosaccharide content (r = 0.75), total soil protein content (r = 0.995***), total soil phenolic acid content (r = 0.997***), and alkaline extractable humic substance content (r = 0.98**). Alkaline extractable humic substances were correlated with the phenolic acid content of the humic substances (r = 0.996***). The results suggest that the decrease in soil stability after soybean growth was due to a decrease in the content of soil humic substances caused by the substantially lower phenolic acids content (humic acid precursors) in the soybean residue.

The Angular Momentum Evolution of Very Low Mass Stars

We present theoretical models of the angular momentum evolution of very low mass stars (0.1-0.5 M☉). We also present models of solar analogs (0.6-1.1 M☉) for comparison with previous work. We investigate the effect of rotation on the effective temperature and luminosity of these stars. Rotation lowers the effective temperature and luminosity of the models relative to standard models of the same mass and composition. We find that the decrease in Teff and L can be significant at the higher end of our mass range but becomes small below 0.4 M☉. The effects of different assumptions about internal angular momentum transport are discussed. Formulae for relating Teff to mass and vrot are presented. We demonstrate that the kinetic energy of rotation is not a significant contribution to the luminosity of low-mass stars. Previous studies of the angular momentum evolution of low-mass stars concentrated on solar analogs and were complicated by uncertainties related to the internal transport of angular momentum. In this paper we extend our theoretical models for the angular momentum evolution of stars down to 0.1 M☉. We compare our models to rotational data from young open clusters of different ages to infer the rotational history of low-mass stars and the dependence of initial conditions and rotational evolution on mass. We find that the qualitative conclusions for stars below 0.6 M☉ do not depend on the assumptions about internal angular momentum transport with the exception of a zero-point shift in the angular momentum loss saturation threshold. We argue that this makes these low-mass stars ideal candidates for the study of the angular momentum loss law and distribution of initial conditions. For stars with masses between 0.6 and 1.1 M☉, scaling the saturation threshold by the Rossby number can reproduce the observed mass dependence of the stellar angular momentum evolution. We find that neither models with solid-body rotation nor differentially rotating models can simultaneously reproduce the observed stellar spin-down in the 0.6-1.1 M☉ range and for stars between 0.1 and 0.6 M☉. We argue that the most likely explanation is that the saturation threshold drops more steeply at low masses than would be predicted with a simple Rossby scaling. In young clusters there is a systematic increase in the mean rotation rate with decreased temperature below 3500 K (0.4 M☉). This suggests either inefficient angular momentum loss or mass-dependent initial conditions for stars near the fully convective boundary.

Geological constraints on the Precambrian history of Earth's rotation and the Moon's orbit

Over the past decade the analysis of sedimentary cyclic rhythmites of tidal origin, i.e., stacked thin beds or laminae usually of sandstone, siltstone, and mudstone that display periodic variations in thickness reflecting a strong tidal influence on sedimentation, has provided information on Earth's paleorotation and the evolving lunar orbit for Precambrian time (before 540 Ma). Depositional environments of tidal rhythmites range from estuarine to tidal delta, with a wave‐protected, distal ebb tidal delta setting being particularly favorable for the deposition and preservation of long, detailed rhythmite records. The potential sediment load of nearshore tidal currents and the effectiveness of the tide as an agent of sediment entrainment and deposition are related directly to tidal range (or maximum tidal height) and consequent current speed. Hence the thickness of successive laminae deposited by tidal currents can be a proxy tidal record, with paleotidal and paleorotational values being determined by analysis of measured records of lamina and cycle thickness. The validity of the findings can be investigated by testing the primary, observed values for internal self‐consistency through application of the laws of celestial mechanics. Paleotidal and paleorotational values provided by late Neoproterozoic (∼620 Ma) tidal rhythmites in South Australia are validated by these tests and indicate 13.1±0.1 synodic (lunar) months/yr, 400±7 solar days/yr, a length of day of 21.9±0.4 h, and a relative Earth‐Moon distance a/a0 of 0.965±0.005. The mean rate of lunar recession since that time is 2.17±0.31 cm/yr, which is little more than half the present rate of lunar recession of 3.82±0.07 cm/yr obtained by lunar laser ranging. The late Neoproterozoic data militate against significant overall change in Earth's moment of inertia and radius at least since 620 Ma. Cyclicity displayed by Paleoproterozoic (2450 Ma) banded iron formation in Western Australia may record tidal influences on the discharge and/or dispersal of submarine hydrothermal plumes and suggests 14.5±0.5 synodic months/yr and a/a0 = 0.906±0.029. The combined rhythmite data give a mean rate of lunar recession of 1.24±0.71 cm/yr during most of the Proterozoic (2450–620 Ma), suggesting that a close approach of the Moon did not occur during earlier time. Concentrated study of Precambrian tidal rhythmites promises to illuminate the evolving dynamics of the early Earth‐Moon system and may permit the lunar orbit to be traced back to near the time of the Moon's origin.

Transfer of Phosphate Ions between Soil and Solution: Perspectives in Soil Testing

AbstractPhosphorus inputs must be estimated accurately to optimize the economic return to farmers and minimize P loss from soils to surface waters. Currently, P recommendations are based on the diagnosis of field crop responses by chemically extracted soil P. However, the inability of chemical extraction to characterize plant‐available P limits the reliability of these recommendations. Major sources of P mobilized by plant roots include P ions in solution and those from soil constituents, which replenish and buffer solution. A mechanistic evaluation of soil P supply should therefore be based on the description of P ion transfer between soil constituents and solution. Sorption, desorption, electro‐ultrafiltration (EUF), and isotopic exchange studies show that an adequate modeling of this quantity [Q(CP,t)] of P ions must account for both the concentration of P ions in soil solution (Cp) and time (t). In one long‐term field experiment, the Q(CP,t) description was not affected by crop rotation and mineral fertilization histories; therefore, Q(CP,t) changes are fully explained by CP changes. In two field experiments, CP changes were linearly correlated with the cumulative P budget, inputs, and outputs over years. In three field experiments, the soil type effect on the relative maize (Zea mays L.) response curve was taken into account using the ability of soil P to replenish solution P for 1 d. The residual variance of this diagnosis is halved compared to Olsen's extraction. Although more information is necessary, accuracy is improved when soil testing is based on mobility of P ions.

IMPACT OF TILLAGE AND TRACER APPLICATION METHOD ON SPATIAL DISTRIBUTION OF LEACHING LOSSES

The impact of tillage and nitrogen (N) application method on the spatial variation on leaching losses wasevaluated using eight, 1-m3 undisturbed soil monoliths. Monoliths were collected in 1992 from research plots with 15years of continuous tillage and crop rotation history. Tillage practices imposed in the field were moldboard plow, chisel,or ridge tillage. Anion tracers were applied to simulate applying N as a surface broadcast, in a slot with surfacecompaction, and with water. A rainfall simulator was used to apply 100 mm of water followed 24 h later by an additionalapplication of 430 mm to establish tracer concentrations over a range of drainage. Drainage was collected at the bottomof each monolith using fiberglass wick extractors placed in a 6 6 grid of 90 mm 90 mm cells. Tracer leaching lossesand flow-weighted concentrations were calculated from the initial flush of water through 24 h after water application. Nosignificant differences were noted for cumulative drainage distribution curves or for the depth of drainage produced (p <0.05). However, in all cases, the cumulative distribution curves were above the 1:1 line, indicating that drainage fromsome cells was greater than others. Spatial analysis indicated that drainage was randomly distributed across themonolith. Tracer leaching losses were not significantly different among tillage treatments or tracer application methodsfor either water application event. However, results for the slot with surface compaction treatment suggest that 10 timesmore NO3-N from moldboard plow treatment in comparison with the ridge tillage treatment. Trends in leaching losses forthe SLOT (Br) with surface compaction treatment suggested that a tracer leaching pattern existed directly below theapplication zone. Tracer concentrations peaked above 350 mg L1 after 100 to 170 mm of drainage for the SLOT (Br)with surface compaction application method and final concentrations remained above 70 mg L1. Peak concentrations forthe surface broadcast (BROAD) and with water application (WATER) methods peaked at less than 110 mg L1 and wereconsistent among monoliths. Spatial analysis indicated that leaching losses were randomly distributed. Data supported anassertion that the moldboard plow tillage treatment combined with the slot with surface compaction (SLOT) applicationof N should be avoided.

An MHD Model of Be Stars with Disks

AbstractAn investigation of the formation and evolution of Be stars show that these stars could have dipolar fossil magnetic fields. This magnetic field should constrain the circumstellar magnetic disk and influence its dynamical features. For studying these effects we refine the alphamodel of accretion disks of Shakura and Sunyaev by incorporating into this model the evolution of large-scale magnetic fields. In the frame of our model we can investigate the rotational history of the star with the disk and the ionization and thermal properties of disks. We show in particular that in the magnetopause region a current sheet can form that can generate X-ray radiation.

Low–Mass He WDs and Millisecond Pulsars Age Determination

It is accepted that the formation of a millisecond binary pulsar (MBP) with a low–mass companion may be explained as the end–point of close binary evolution in which an old pulsar is spun–up by accretion from the red giant (Bhattacharaya &amp;amp; van den Heuvel 1991). In this paper we shall discuss the cooling properties of the helium white dwarfs (WD) in short orbital period MBP systems PSR J0437–4715, PSR J0751+1807 and PSR J1012+5307, without referring to the rotational history of neutron stars (NS).Below we discuss observational data for several system for which the results of our calculations (Sarna, Antipova, &amp;amp; Muslimov 1998; Sarna, Ergma, &amp;amp; Antipova 1999) may be applied, by taking into account the orbital parameters of the system, the pulsar spin-down time, and the WD cooling time-scale.

Extensional deformation across an active margin, relations with subsidence, uplift, and rotations: The Hikurangi subduction, New Zealand

On the basis of field studies in the forearc domain of the Hikurangi subduction margin, in the eastern North Island of New Zealand, we analyze the evolution of the subduction since its onset 25 Myr ago. Analyses of brittle deformation within this forearc domain have revealed evidence of extensional deformation, contemporaneous with the subduction of the Pacific plate beneath the North Island. The type and origin of extensional deformation during the development of the active margin are discussed. The problem is crucial because this forearc domain was previously considered as having undergone almost continuous compression during the late Cenozoic. Two distinct events of late Cenozoic extensional deformation are identified. The youngest one is Quaternary in age; it affects limited areas where important uplift prevails. The orientation of extensional paleostress axes is perpendicular to that of uplift axes, hence consistent with the classical hypothesis of gravitational collapse affecting the upper part of the margin. Of particular interest is the older extensional event, middle‐late Miocene in age. It affected most of the forearc domain during a long time span that was also characterized by widespread margin subsidence. Some structures attributed to this event are syndepositional, including rollover structures, and there are mainly high‐angle normal faults that affect not only the Miocene sediments but also the pre‐Miocene basement. The orientations of paleostress axes obtained by inversion of fault data sets are multiple; after corrections taking into account the rotational history of the margin during the late Cenozoic, they are found to be generally consistent with two major trends. Because these extensional structures developed throughout a long period of the margin subsidence, it is proposed that they reflect the process of tectonic erosion that affected the Hikurangi active margin during the middle and late Miocene, between about 15 and 5 Myr ago.

Miocene intra‐arc bending at an arc–arc collision zone, central Japan: Reply

Abstract Recent paleomagnetic studies are reviewed in an effort to clarify the relationship between the intra-arc deformation of central Japan and the collision tectonics of the Izu-Bonin Arc. The cusp structure of the pre-Neogene terranes of central Japan, called the Kanto Syntaxis, suggests a collisional origin with the Izu-Bonin Arc. The paleomagnetic results and newly obtained radiometric ages of the Kanto Mountains revealed the Miocene rotational history of the east wing of the Kanto Syntaxis. More than 90° clockwise rotation of the Kanto Mountains took place after deposition of the Miocene Chichibu Basin (planktonic foraminiferal zone of N.8: 16.6–15.2 Ma). After synthesizing the paleomagnetic data of the Japanese Islands and collision tectonics of central Japan, it appears that approximately a half rotation (40–50°) probably occurred at ca 15 Ma in association with the rapid rotation of Southwest Japan. The remainder (50-40°) continued until 6 Ma, resulting in the sharp bent structure of the pre-Neogene accretionary complexes (Kanto Syntaxis). The latter rotation seems to have been caused by the collision of the Izu-Bonin Arc on the northwestward migrating Philippine Sea Plate.

Plate tectonic reconstructions of the Juan Fernandez microplate: Transformation from internal shear to rigid rotation

Side‐scan sonar, swath bathymetry and magnetic anomaly date define a detailed, three‐phase history of the Juan Fernandez microplate. The ∼6 m.y. history is presented in a series of discrete time steps to document the growth and reorganization of propagating spreading centers and structural feature, and microplate kinematic evolution. Prior to the microplate, the East Pacific Rise at the Pacific‐Antarctic‐Nazca triple junction was offset by a long transform fault zone, likely the fastest slipping transform on Earth at anomaly 3A time. The microplate originated from an intratransform setting between anomaly 3A (5.95 Ma) and anomaly 3 (5.24 Ma) time. Its early development resembled a large propagating rift system, and microplate core structures suggest the entire offset zone may have experienced deformation. Fast propagation of the East Ridge dominated microplate growth until ∼2.6–1.9 Ma when seafloor spreading became the dominant growth process. The microplate rotation rate increased threefold (from 9 to 29° m.y.−1 average) from phase 1 (4.2–2.6 Ma) to phase 2 (2.6–1.1 Ma) of the microplate's history, then reduced fourfold (29 to 7° m.y. −1 average; phase 3, 1.1 Ma to Present). Phases 2 and 3 of the microplate's rotational history support the edge‐driven model for microplate kinematics of Schouten and others to a good approximation. The Pacific‐Nazca shear couple drove microplate rotation during phase 2, but development of the southeastern boundary enabled a transfer to the Nazca‐Antarctic plate pair (phase 3). West Ridge propagation and reorganization of the south‐western boundary may have decoupled the Pacific plate from the microplate, thus facilitating the shear couple transfer. The recent continued deceleration in microplate rotation rate and westward migration of the Pacific‐Antarctic ridge axis relative to the microplate may indicate that the process of microplate “death” has begun. We speculate that the Juan Fernandez microplate will accrete to the Antarctic plate, perhaps within the next million years, like the extinct Friday microplate has done, thereby accomplishing another northward migration of the Pacific‐Antarctic‐Nazca triple junction. Our reconstructions illustrate that the Easter and Juan Fernandez microplates are more similar than previously thought in terms of their origin, growth, rift propagation, ridge segmentation and overall tectonic evolution.

THE IMBEDDING EQUATIONS FOR THE TIMOSHENKO BEAM

Wave reflection in a Timoshenko beam is treated, using wave splitting and the imbedding technique. The beam is assumed to be inhomogeneous and restrained by a viscoelastic suspension. The viscoelasticity is characterized by constitutive relations that involve the past history of deflection and rotation of the beam through memory functions of the suspension. By applying wave splitting, the propagating fields are decomposed into left-and right-moving parts. An integral representation of the split fields in impulse responses is presented. This representation gives the reflected and transmitted fields as convolutions of the incident field with the reflection and transmission kernels, respectively. The kernels are independent of the incident field and depend only on the material properties. Invariant imbedding is used to obtain equations for these kernels. In general, the kernels contain discontinuities for which transport equations are derived and solved. Some numerical solutions are presented for the reflection by a homogeneous beam suspended on two separated, semi-infinite layers of continuously distributed, viscoelastically damped, local acting springs.

Trajectory Matching Flight-Path Optimization of Aerospace Vehicles

A new class of six-degree-of-freedom optimization problems referred to as trajectory matching e ight-path optimization is addressed and explicitly described via an example problem. The essence of this class of e ightpath optimization problems is to force an aerospace vehicle to follow prescribed translational and rotational velocity histories or to motion-match. For this example, a lightweight, axisymmetric re-entry vehicle decoy is designed to match the motion of a typical full-weight re-entry vehicle by optimizing its mass properties. The optimization problem, which is performed in two steps, is given as the minimization of the difference between the target trajectory and the actual trajectory subject to physical constraints. The e rst step utilizes a unique stability measure to develop similarity parameters that provide excellent initial guesses for the numerical optimization routine employed in the second step. The second step utilizes a sequential quadratic programming algorithm to numerically solve the constrained minimization problem. Simulation results demonstrate the robust trajectory matching of the lightweight re-entry vehicle.

Miocene intra‐arc bending at an arc‐arc collision zone, central Japan

Abstract Recent paleomagnetic studies are reviewed in an effort to clarify the relationship between the intra‐arc deformation of central Japan and the collision tectonics of the Izu‐Bonin Arc. The cusp structure of the pre‐Neogene terranes of central Japan, called the Kanto Syntaxis, suggests a collisional origin with the Izu‐Bonin Arc. The paleomagnetic results and newly obtained radiometric ages of the Kanto Mountains revealed the Miocene rotational history of the east wing of the Kanto Syntaxis. More than 90° clockwise rotation of the Kanto Mountains took place after deposition of the Miocene Chichibu Basin (planktonic foraminiferal zone of N.8: 16.6–15.2 Ma). After synthesizing the paleomagnetic data of the Japanese Islands and collision tectonics of central Japan, it appears that approximately a half rotation (40–50°) probably occurred at ca 15 Ma in association with the rapid rotation of Southwest Japan. The remainder (50‐40°) continued until 6 Ma, resulting in the sharp bent structure of the pre‐Neogene accretionary complexes (Kanto Syntaxis). The latter rotation seems to have been caused by the collision of the Izu‐Bonin Arc on the northwestward migrating Philippine Sea Plate.

Defining relations for a viscoelastic medium with microrotation

The results of [1, 2] are extended to the case of a Cosserat medium with a memory (the force stress tensor and the couple stress tensor depend on the history of deformations and rotations of a particle in the medium). In the linear approximation the defining relations have the form of convolutions with some relaxation kernels with respect to time. Restrictions for the kernels are obtained, which follow from the general principles of thermodynamics. The propagation of weak perturbations is studied. The general functional form of the ken nels corresponding to experimental data on the viscoelasticity of rock formations is given.

Cenozoic SE Asia: reconstructing its aggregation and reorganization

Abstract Reconstructing SE Asia in the Cenozoic is dependent on being able to model three major tectonic events and their consequences. They are the collision of India with Eurasia, the rotational history of the Philippine Sea plate and the ongoing collision of Australia with eastern Indonesia. Models of the Eocene India-Eurasia collision imply extrusion along major strike-slip faults or crustal thickening and block rotation. Kinematic modelling of the present day tectonics of central Asia indicates rotations and strike-slip faulting and an eastward displacement of crust. Prior to middle Miocene time tectonic activity was closely associated with the collision zone and extrusion of Indochina was supposed to have occurred. Analysis of the volume of the topography formed by the collision indicates eastward displacement and the loss of some crust by expulsion or into the mantle. Palaeomagnetic measurements have found northward displacement of Tibet and strong clockwise rotation east of the eastern syntaxis but unequivocal evidence of southward displacement is rare. Large Palaeogene to early Neogene displacements on strike-slip faults have been documented and related with confidence to the rotations that formed at least the Oligocene South China Sea floor. Early Oligocene docking of the Indoburman Flysch belt south of the eastern syntaxis is difficult to explain in extrusion models. Whether the rotations that have been documented are a response by internal deformation or block rotation is unresolved, as is the amount of extrusion of SE Asia to the south. In the eastern region, rotation and associated northward motion of the Philippine SEa plate as it developed puts constraints on possible models because of a maintained close proximity of its southern margin to the northern edge of the Australian craton. Assembling of continental fragments now in the Banda Sea and derived from eastern New Guinea appears to have occurred by Late Cretaceous time. Formation of the Banda arc may have isolated the Banda Sea from the Australia plate then collision of the Australian margin possibly rotated the Banda Sea counter-clockwise. Although westward movement of the Philippine Sea plate relative to Australia commenced in the early Miocene, convergent left-lateral shear was not possible until the late Miocene thus limiting westward tectonic transport of terranes and the transmission of the shear into the region west of the Banda Sea.

New light on old problems of geology

In studying the problem of the origin and history of rotation of the Sun and the planets, the author has concluded that the present speed of the Earth's rotation has increased during geological history. Therefore, material (physical) evidence of this phenomenon should exist in the planet's interior. This conclusion is fully confirmed by the vast amount of information contained in the Earth's sedimentary cover.

Origin and motion history of the Philippine Sea Plate

The Philippine Sea Plate is the one major plate whose Tertiary motion is poorly constrained and whose origin is problematical. Its southern boundary is the Sorong Fault system which is part of a major left-lateral fault system at the northern margin of the Australian plate. The southern part of the plate in eastern Indonesia has been neglected in most syntheses but includes some of the oldest rocks within the plate which are separated from remnant arcs of the Daito Ridge province of the northern Philippine Sea by the West Philippine Central Basin. The east Indonesian islands of the Halmahera-Waigeo region contain a good Mesozoic and Tertiary stratigraphic record indicating a long arc history for the southern part of the plate. New palaeomagnetic data from these islands define two sub-areas: an area forming part of the Philippine Sea Plate north of the Sorong Fault, and an area within the Sorong Fault system. The area north of the fault records a long-term clockwise rotation history whereas that within the fault zone records local rotations interpreted as due to deformation at the plate edge. Rocks of Philippine Sea Plate origin within both areas record similar latitudinal shifts. The rotation of the area north of the Sorong Fault is considered to represent the motion of the southern part of the Philippine Sea Plate. The new data indicate large Tertiary clockwise rotations similar to earlier suggestions for other parts of the plate but record a discontinuous and more complex motion history than previously suggested. For the southern part of the plate there was 40° rotation with northward translation between 0 and 25 Ma, no significant rotation between 25 and 40 Ma, and there was 50° rotation with southward translation between 40 and 50 Ma. We show that the new palaeomagnetic data form part of a single set with earlier palaeomagnetic data from elsewhere in the plate. The translation history of the southern part of the plate in eastern Indonesia can be reconciled with northward motions recorded elsewhere and can be used to determine rotation poles for the plate (15°N, 160°E for the interval 5–25 Ma, and 10°N, 150°E for the interval 40–50 Ma). Reconstructions based on these poles predict that at ∼ 45 Ma the Palau-Kyushu Ridge had a WNW-ESE orientation which is very different from that postulated by many models used to explain the widespread boninite volcanism in the Izu-Bonin-Marians forearc at this time. The long arc history of the southern part of the plate and the reconstructions based on the rotation poles calculated from the palaeomagnetic data favour an origin for the West Philippine Basin by spreading in a backarc basin.

Evolution of the boundary between the Philippine Sea Plate and Australia: palaeomagnetic evidence from eastern Indonesia

The boundary between the Philippine Sea and Australian plates is the left-lateral Sorong Fault system of eastern Indonesia. Until recently, modelling this boundary for the period before about 5 Ma was difficult; the Tertiary motion of the Philippine Sea Plate was uncertain and palaeomagnetic data from areas adjacent to the fault were lacking. Recent geological and palaeomagnetic studies of the area north of the Sorong Fault have elucidated the Tertiary motion history of the Philippine Sea Plate, providing a reference for examining movements within the fault system. We report new palaeomagnetic data from within the Sorong Fault Zone, from the islands of Taliabu and Obi. Taliabu is part of the Sula Platform and is considered to be derived from Australia. Pelagic limestones from the Upper Cretaceous Tanamu Formation of Taliabu yielded a direction of D = 329.1°, I = −34.9° implying counter-clockwise rotation and a formation latitude of 19 ± 5°S. Sula and Misool are postulated to be part of a single microcontinent which had a different Late Cretaceous-mid-Tertiary movement history from Australia. The Sula Platform was transported to its present position by movement along the Sorong Fault system in the Late Miocene. Obi includes rocks of Philippine Sea and Australian origin; all the new sites are in rocks of Philippine Sea Plate origin. Since the Early Neogene the Philippine Sea Plate, which includes all islands north of the Sorong Fault, has rotated 40° clockwise and moved 10–15° northwards. Philippine Sea Plate rocks within the Sorong Fault Zone record similar latitude shifts, but different rotations. In north Obi, the Upper Oligocene Anggai River Formation and the Middle Miocene Woi Formation record ∼ 60° and ∼ 30° counter-clockwise rotations, respectively. The sense of rotation is consistent with motion within a left-lateral fault system, with the Philippine Sea and Australian plates providing the shear couple. In contrast, the Woi Formation in southeast Obi records 15–20° clockwise rotation; this area is separated from the zone of counter-clockwise movement in north Obi by a strand of the Sorong Fault. Arc volcaniclastic rocks from the Upper Cretaceous Leleobasso Formation of northwest Obi have a primary magnetisation with a mean direction of D = 357.1°, I = −21.9°. These rocks formed at ∼ 11°N or ∼ 11°S, depending on the interpreted rotation history, and indicate a Pacific rather than Indian Ocean origin. A volcanic arc at the southern edge of the Philippine Sea Plate collided with eastern New Guinea at ∼ 25 Ma. The Philippine Sea-Australia plate boundary then changed from subduction to strike-slip, as the Philippine Sea Plate began its Neogene rotation, initiating the Sorong Fault system. We suggest that many of the arc fragments in the New Guinea orogenic belt originated in the southern Philippine Sea Plate arc which has subsequently been dismembered by strike-slip faulting.

Paleomagnetic analysis of fold kinematics and implications for geological models of the Cantabrian/Asturian arc, north Spain

The Ponga Unit lies in the core of the Cantabrian/Asturian arc, a thrust system which was emplaced and then noncoaxially refolded during the Variscan Orogeny. Alternative models for thrust system geometry prior to refolding of the Ponga Unit have been proposed. Although the Ponga Unit becomes populated with steeply plunging folds under each model, the alternative models imply differing kinematic histories for these folds. Paleomagnetic data were acquired from 22 sites in grey and red Carboniferous limestones around these folds in order to determine the rotational histories of the structures and hence to identify the appropriate geological model for initial thrust system geometry. Low anisotropy of magnetic susceptibility indicated that the samples were suitable for rigid‐body rotation studies. Four remanence components were identified on stepwise demagnetization: A very low temperature, randomly orientated component V; a low‐temperature component L present between 50°C and 200°C (64/349, α95 = 4.4°), clustered around the Recent field in geographic coordinates; an intermediate‐temperature component I present between 200°C and 400–600°C, of reverse polarity and shallow inclination and showing significant between‐site dispersion; and a high‐temperature component H (30/342, α95 = 21°), lying close to the Eocene reference direction in geographic coordinates. The I component fails a fold test based upon a first deformation phase (D1) hanging wall anticline and is therefore a remagnetization. The I component consistently gives positive fold tests of the steeply plunging fold structures when the structures are unfolded about their plunging hinges. This demonstrates that these folds result from north‐south shortening of essentially homoclinal westerly dipping D1 hanging wall ramps and that alternative models involving steeply dipping interference folds arising from complex hanging wall deformation during D1 are invalid. A partial restoration of the thrust system based on the former model with the paleomagnetic data pinned to a calculated Stephanian reference direction shows that the thrust front was approximately north‐south in trend in the Ponga Unit, as it was farther south in the Esla Unit, where anticlockwise rotations and dextral wrench faulting have subsequently occurred. The data also record a tilt to the north of up to 30°, which has occurred incrementally since the end of Variscan deformation.

Low-Cycle Variable Amplitude Fatigue Modeling of Top-and-Seat Angle Connections

A set of variable amplitude tests was performed on a commonly used type of semi-rigid connection: the top-and seat angle. Variable amplitude tests included incremental decremental and step tests to investigate the sequence effects on cyclic and low-cycle fatigue behavior for this class of beam column connection. The concept of an effective equi-amplitude plastic rotation is introduced as the characteristic measure of rotation history. It is shown that effective rotation is independent of load path when compared to companion constant amplitude test results. An energy based cumulative damage model is characterized in terms of energy-life and energy-rotation relationships. The proposed damage model is employed in fatigue damage prediction analysis and compared with the classical Miner's linear damage accumulation model coupled with the rainflow cycle counting method. The proposed energy based damage model implicitly incorporates the memory effects of the previous loading history and precludes the necessity of performing explicit cycle counting, and it is easily implemented into non-linear time-history analysis for seismic design and evaluation purposes.