Low Flexural Rigidity Research Articles

Quenching of vortex-induced vibrations of towering structure and generation of electricity using Hula-Hoops

This paper deals with the quenching problem of vortex-induced vibrations by using devices composed of Hula-Hoops and generators. These devices are also able to generate electricity. The experiment was made concerning the quenching problem of the vortex-induced vibrations of towering structures with the devices. Moreover, a numerical analysis was also carried out using the Runge–Kutta–Gill method. An optimum approach for quenching the vortex-induced vibrations of low and high flexural rigidity directions are discussed. As a result, the following was made clear: (1) The vortex-induced vibrations of both flexural rigidity directions were well quenched for a wide range of wind velocities and the different wind directions by using four devices simultaneously. (2) The optimally quenched vibration was a chaos of intermittency. (3) A condition of maximum power generation was not equal to that of the optimum vibration quenching of the main system. (4) The experimental and the analytical results were in good qualitative agreement with each other.

Modeling postbreakup landscape development and denudational history across the southeast African (Drakensberg Escarpment) margin

We employ a numerical surface processes model to study the controls on postbreakup landscape development and denudational history of the southeast African margin. Apatite fission track data, presented in the companion paper, suggest that the Drakensberg Escarpment formed by rapid postbreakup river incision seaward of a preexisting drainage divide, located close to its present position, and subsequently retreated at rates of only ∼100 m m.y.−1. Numerical modeling results support such a scenario and show that the prebreakup topography of the margin has exerted a fundamental control on subsequent margin evolution. The rheology of the lithosphere, lithological variations in the eroding upper crust, and inland base level falls provided secondary controls. A relatively low flexural rigidity of the lithosphere (Te ≈ 10 km) is required to explain the observed pattern of denudation as well as the observed geological structure of the southeast African margin. Lithological variations have contributed to the formation of flat‐topped ridges buttressing the main escarpment, as well as major fluvial knickpoints. Both these features have previously been interpreted as supporting significant Cenozoic uplift of the margin. An inland base level fall, possibly related to back‐cutting of the Orange River drainage system and occurring 40–50 m.y. after breakup, explains the observed denudation inland of the escarpment as well as the development of inland drainage parallel to the escarpment. Our model results suggest that in contrast to widely accepted inferences from classical geomorphic studies, the southeast African margin has remained tectonically stable since breakup and escarpment retreat has been minimal (<25 km).

Evidence of low flexural rigidity and low viscosity lower continental crust during continental break-up in the South China Sea

The South China Sea was formed by seafloor spreading in the Late Oligocene at ∼30 Ma following a series of extensional events within crust formed by Mesozoic continental arc. In this study, we interpreted faults along seismic reflection profiles from both the northern and southern conjugate margins of the South China Sea, and forward modeled these using a flexural cantilever model to predict modern basin geometries. When compared with the observed structure, the models based on upper crustal faulting consistently underpredicted the amount of subsidence, especially towards the continent–ocean transition (COT). We interpret this to indicate preferential extension of the continental lower crust along the COT on both margins, extending up to ∼80 km landward from COT. The regional slope of the South China continental shelf indicates lower crustal viscosities of 10 19–10 18 Pa s, representing an offshore continuation of the weak crust documented onshore on the eastern flanks of the Tibetan Plateau. Only in the region of Hainan Island in the western South China Shelf does lower crustal viscosity increase (10 21–10 22 Pa s) and the preferential loss of lower crust become less pronounced and limited to <40 km from COT. This western area represents a rigid block analogous to the Sichuan Basin onshore. Forward models based on upper crustal faulting support the idea of a very weak continental crust because models where the effective elastic thickness of the plate ( T e) exceeds 5 km fail to reproduce the geometry of the sub-basins within the Pearl River Mouth Basin (PRMB) of the South China Margin. The observed basins are too deep and narrow to be consistent with models invoking high flexural rigidity in the upper crust or mantle lithosphere. The fact that rifting and seafloor spreading seem to co-exist for ∼5 my. adjacent to the PRMB is consistent with very weak continental crust during break-up.

Timing and mechanisms of North Atlantic Cenozoic uplift: evidence for mantle upwelling

Abstract Postrift domal uplift patterns are a distinct feature of northern North Atlantic margins. On the basis of apatite fission-track data, offshore seismic stratigraphy, geomorphology, gravity and seismic tomography, we argue that southern Norway is characterized by predominantly Neogene domal uplift. The uplift is tectonically driven and estimated at around 1.5 km. Low flexural rigidity ( c . 10 22 N m) and corresponding equivalent elastic thickness T e ( c . 15 km) values for the southern Norwegian lithosphere indicate that the lithosphere is relatively weak. Additionally, high temperature estimates derived from low-velocity mantle P- and S-wave seismic tomography below the dome suggest a thermal anomaly at depth. Therefore, the observed topography is most plausibly explained by mantle upwelling. This would supercede other previously proposed primary mechanisms such as eustasy, isostatic readjustment to glacial erosion, magmatic underplating and intraplate compression. Currently available data suggest similar processes for other uplifted regions such as Spitsbergen, northern Norway, the British Isles and parts of East Greenland.

Crustal structure and thermal anomalies of the Dunedin Region, South Island, New Zealand

Seven geophysical data sets are used to investigate a transect along the southeast coast of South Island, New Zealand. The specific focus of this study is the Dunedin volcanic center, which last produced volcanics at the surface 13–10 Myr ago. Wide‐angle reflection/refraction seismic data along a two‐dimensional profile reveal a low‐velocity lower crust and mantle beneath the Dunedin volcanic center. The low‐velocity lower crust coincides with a highly reflective region on a nearby multichannel seismic line and may represent a hot, fluid‐rich region of the crust. In addition, high mantle helium ratios measured in the Dunedin region suggest a current or recent mantle‐melting event. High heat flow recorded in the Dunedin region is consistent with a hot body emplaced in the midcrust ∼10 Myr ago (Miocene) whose heat is just reaching the surface today. Uplift of an Oligocene limestone horizon in the Canterbury basin can be explained by a buoyant load beneath the Dunedin volcanic center and low flexural rigidity of the lithosphere beneath the volcanic center during the Miocene. We interpret the data as revealing two separate thermal events beneath the Dunedin volcanic center, one during the Miocene, when volcanism was last occurring at the surface, and the other occurring currently. Active volcanism associated with the current mantle‐melting event has yet to reach the surface.

Kinematics of oblique collision and ramping inferred from microstructures and strain in middle crustal rocks, central Southern Alps, New Zealand

The hanging wall of the Alpine Fault near Franz Josef Glacier has been exhumed during the past ∼2–3m.y. providing a sample of the ductilely deformed middle crust of a modern obliquely convergent orogen. Presently exposed rocks of the Pacific Plate are inferred to have undergone several phases of ductile deformation as they moved westward above a mid-crustal detachment. Initially they were transpressed across the outboard part of the orogen, resulting in oblate fabrics with a down-dip stretch. Later, they encountered the Alpine Fault, experiencing an oblique-slip backshearing on vertical planes. This escalator-like deformation tilted and thinned the incoming crust onto that crustal-scale oblique ramp. This style of hanging wall deformation may affect only the most rapidly uplifting, central part of the Southern Alps because of the low flexural rigidity of the crust in that region and its displacement over a relatively sharp ramp-angle at depth. A 3D transpressive flow affected mylonites locally near the fault, but their shear direction remained parallel to plate motion, ruling out ductile ‘extrusion’ as an important process in this orogen. Outside the mylonite zone, late Cenozoic shortening is inferred to be modest (30–40%), as measured from deformation of younger biotite grains. Oblique collision is dominated by translation on the Alpine Fault, and rocks migrate rapidly through the deforming zone, preventing the accumulation of large finite strains. Transpression may play a minor role in oblique collision.

MANUFACTURE, TESTING AND NUMERICAL ANALYSIS OF AN INNOVATIVE POLYMER COMPOSITE/CONCRETE STRUCTURAL UNIT.

Keywords SURREY UNIVERSITY ADVANCED COMPOSITE GROUP CONCRETE, COMPOSITE, STRUCTURAL, UNITS, POLYMERS, INNOVATIVE, MANUFACTURE, MANUFACTURING, TESTING, NUMERICAL, ANALYSIS, TESTS, NUMERICAL, ANALYSIS, TESTING, NUMERICAL, ANALYSIS, TESTS, BEAMS, DUPLEX, LOADS, EXPERIMENTS, BENDING, FINITE ELEMENTS, COMPRESSIVE, GFRP, GLASS, FIBRES, REINFORCED, SANDWICH, FLANGES, ACTION, INDENTS, FOAM, SHUTTERING, FAILURES, FLEXURAL, CRUSHING, WEBS, SHEAR, BEHAVIOUR, PREDICTION, RIGIDITY, CAPACITY, REINFORCEMENT, STRENGTH, MATERIALS, PREPREG... Show All

The lithosphere under stress

I discuss thermal, rheological and compositional definitions of the Earth's lithosphere and describe the data that can be brought to bear in refining these definitions. Models of the behaviour of t...

Special Issue on Handling of Flexible Object

Special Issue on Handling of Flexible Object

The East Carpathians: Indications of phase transitions, lithospheric failure and decoupled evolution of thrust belt and its foreland

An analysis is presented of the tectonic development of the East Carpathians since the Late Oligocene. The geological and geomorphological data show that five epochs of convergence and nappe emplacement took place with the last one occurring 11–12 m.y. B.P. After these compressional events the nappe surface remained near sea level. The crustal uplift which formed the present mountains began 3 m.y. B.P. and after 99% of the shortening of the Carpathians had been completed. The Pre-Carpathian foreland basin, up to 3–5 km deep, formed on the margin of the East European Platform in the Middle Miocene. Analysis of the evolution of the basin shows that it deepened away from the nappe-stack and most of the subsidence occurred when there was no convergence. This precludes lithospheric flexure due to convergence as a mechanism for basin formation. At 11–12 m.y. B.P. the inner part of the basin, ∼ 50 km wide, was overridden by the Carpathian nappe, up to 12–14 km thick. At that time, a slight crustal uplift took place, rather than subsidence, in the shallow remnant-outer basin part (the present foredeep) which emerged above sea level. An additional 7–8 km of subsidence then occurred in the foredeep of the southeastern Carpathians in the Late Miocene-Early Pleistocene when very little convergence took place. The evolution of the Carpathian foredeep shows that lithospheric flexure of 2–5 km occurred in regions, 20–40 km wide. Similar amounts of flexure and large basement faults exist under the nappe. This style of deformation indicates flexural rigidity of the lithosphere that is several orders of magnitude smaller than in stable cratonic regions. Under a low flexural rigidity, the crust should be close to local isostasy. Then the isostatic response to superposition of a 10–13-km-thick nappe on the platform margin which was near sea level would form high mountains. The crustal surface, however, remained at a low altitude. We interpret this to be the result of contraction of gabbro in the lower crust with the formation of dense garnet granulites during nappe emplacement. A layer of these dense rocks, ∼ 20 km thick, with P-wave velocities 7.5–7.6 km/s, exists in the lower crust of the East Carpathians. According to the geophysical data, there is no cool high-velocity slab now subducting towards the Intra-Carpathian area. An increase in the mean height of the Carpathians by only ∼ 1 km since the termination of convergence precludes a recent slab detachment. Rapid metamorphism of mafic rocks in the lower crust requires the infiltration of volatiles from the asthenosphere. Infiltration of volatiles into mantle lithosphere and the upper crust could strongly reduce their viscosity. These phenomena might be the cause of an abrupt loss of lithospheric rigidity on the margin of the East European Platform in the Neogene. Lithospheric failure allowed the incorporation of this region into the Carpathian fold belt after a long period of cratonic development.

Noise from Separated Airflow on a Panel. 1st Report. Generation and Transmission Mechanisms of Noise from Vibration of a Panel.

Aerodynamic noise generation and the transmission mechanisms caused by airflow past a fence on a flat panel were investigated experimentally. Distinctively different noise sources were respectively identified in cases of high and low flexural rigidity of the panel : (a) In the case of high flexural rigidity, noise is predominantly generated from the separated airflow behind the fence, and (b) in the case of low flexural rigidity, noise is predominantly due to the vibration of the panel.

Glaciation, erosion and the evolution of the Transantarctic Mountains, Antarctica

Modelling studies of the tectonic evolution of the Transantarctic Mountains in Antarctica have drawn differing conclusions as to the primary mechanisms involved. None has considered the role of the East Antarctic ice sheet in detail. We use a denudation—flexural model to examine the isostatic response of the continental margin to glacial erosion to determine whether glacial processes have played a role in forcing mountain uplift.The conclusion is that, although there are insufficient data formally to delimit the role of glacial erosion, available geophysical and geomorphological data are not inconsistent with the results of the differential denudation model, providing certain conditions are met. These results indicate that the current topography of the Transantarctic Mountains can be simulated, in part, from the isostatic response of the lithosphere to glacial erosion. The short wavelength and high amplitude of the Transantarctic Mountains do not require a low flexural rigidity in the unrated lithosphere, provided there is a fast escarpment retreat from the rift hinge, high escarpment denudation rates and a large differential in denudation between the coastal zone and the interior.

Glaciation, erosion and the evolution of the Transantarctic Mountains, Antarctica

Modelling studies of the tectonic evolution of the Transantarctic Mountains in Antarctica have drawn differing conclusions as to the primary mechanisms involved. None has considered the role of the East Antarctic ice sheet in detail. We use a denudation—flexural model to examine the isostatic response of the continental margin to glacial erosion to determine whether glacial processes have played a role in forcing mountain uplift. The conclusion is that, although there are insufficient data formally to delimit the role of glacial erosion, available geophysical and geomorphological data are not inconsistent with the results of the differential denudation model, providing certain conditions are met. These results indicate that the current topography of the Transantarctic Mountains can be simulated, in part, from the isostatic response of the lithosphere to glacial erosion. The short wavelength and high amplitude of the Transantarctic Mountains do not require a low flexural rigidity in the unrated lithosphere, provided there is a fast escarpment retreat from the rift hinge, high escarpment denudation rates and a large differential in denudation between the coastal zone and the interior.

Flexural rigidity of the Basin and Range‐Colorado Plateau‐Rocky Mountain transition from coherence analysis of gravity and topography

Stochastic inversion for flexural loads and flexural rigidity of the continental elastic layer can be accomplished most effectively by using the coherence of gravity and topography. However, the spatial resolution of coherence analysis has been limited by use of two‐dimensional periodogram spectra from very large (&gt;105 km2) windows that generally include multiple tectonic features. Using a two‐dimensional spectral estimator based on the maximum entropy method, the spatial resolution of flexural properties can be enhanced by a factor of 4 or more, enabling more detailed analysis at the scale of individual tectonic features. This new approach is used to map the spatial variation of flexural rigidity along the Basin and Range transition to the Colorado Plateau and Middle Rocky Mountains physiographic provinces. Large variations in flexural isostatic response are found, with rigidities ranging from as low as 8.7×1020 N m (elastic thickness Te = 4.6 km) in the Basin and Range to as high as 4.1×1024 N m (Te = 77 km) in the Middle Rocky Mountains. These results compare favorably with independent determinations of flexural rigidity in the region. Areas of low flexural rigidity correlate strongly with areas of high surface heat flow, as is expected from the contingence of flexural rigidity on a temperature‐dependent flow law. Also, late Cenozoic normal faults with large displacements are found primarily in areas of low flexural rigidity, while deformation fronts of Mesozoic/Tertiary overthrusts occur 0 to 100 km east of the low‐rigidity region. The highest flexural rigidity is found within the Archean Wyoming craton, where evidence suggests that deeply rooted cratonic lithosphere may play a role in determining the distribution of tectonism at the surface.

Escarpment evolution on high‐elevation rifted margins: Insights derived from a surface processes model that combines diffusion, advection, and reaction

Experiments with a surface processes model of large‐scale (1–1000 km) long‐term (1–100 m.y.) erosional denudation are used to establish the controls on the evolution of a model escarpment that is related to the rifting of a continent. The model describes changes in topographic form as a result of simultaneous short‐ and long‐range mass transport representing hillslope (diffusive) processes and fluvial transport (advection), respectively. Fluvial entrainment is modeled as a first‐order kinetic reaction which reflects the credibility of the substrate, and therefore the fluvial system is not necessarily carrying at capacity. One‐dimensional and planform models demonstrate that the principal controls on the evolution of an initially steep model escarpment are (1) antecedent topography/drainage; (2) the timescale (or equivalently a length scale) in the fluvial entrainment reaction; (3) the flexural response of the lithosphere to denudation; and (4) the relative efficiencies of the short‐ and long‐range transport processes. When rainfall and substrate lithology are uniform, a significant amount of discharge draining over the escarpment top causes it to degrade. Only when the top of the model escarpment coincides with a drainage divide can escarpment retreat occur for these conditions. An additional requirement for retreat of a model escarpment without decline is a long reaction time scale for fluvial entrainment. This corresponds to a substrate that is hard to detach by fluvial erosion, and therefore to fluvial erosion that is not transport limited. Continuous backtilting of an escarpment due to flexural isostatic uplift in response to denudational unloading helps maintain the scarp top as a divide. It is essential if the escarpment gradient is to be preserved during retreat in a uniform lithology. Low flexural rigidities promote steep and slowly retreating escarpments. For given rainfall and substrate conditions, the morphology of a retreating model escarpment is determined by the ratio of the short‐range diffusive and long‐range advective transport efficiencies. A low ratio (which is interpreted to correspond to a relatively arid climate and weathering‐limited conditions) promotes steep, sharp‐topped escarpments with straight main slopes, and escarpment retreat occurs over a wide range of height scales. A high ratio (interpreted to correspond to a more humid, temperate climate) produces a convex upper slope, and concave lower slope morphology and only major escarpments are predicted to preserve a high scarp gradient. Lithological contrasts in the model produce more complex morphologies and predict the formation of scarps crowned by an erosionally resistant caprock. However, resistant caprocks are not an essential requirement for model scarps to retreat. We conclude that the inferred controls and model behavior are both consistent with the present‐day morphology of rifted continental margins and with modern conceptual models of landscape evolution.

Rift flank uplift in Tibet: Evidence for a viscous lower crust

Analysis of high‐resolution digital topography across 21 Plio‐Pleistocene grabens in central and southern Tibet shows that high topography occurs along the graben margins. Profiles across these marginal highlands fit curves describing the flexure of an elastic plate under vertical loading. We interpret the features as flexural rift flank uplifts, resulting from regional isostatic compensation of low‐density grabens. The short wavelengths of the uplifts (30–40 km) imply a low flexural rigidity for the plateau (D = 2−6 × 1020 N m), similar to values inferred for the Basin and Range of western North America. In addition, the grabens themselves are extremely narrow compared to typical continental rifts. Taken together, these observations suggest the existence of a low‐viscosity lower crust and suggest that isostatic compensation is taking place within this ductile lower crust. Simple channel flow calculations suggest that crustal viscosities below ∼1022 Pa s are necessary to accommodate this compensation over reasonable timescales.

Jurassic extension estimates for the North Sea ‘triple junction’ from flexural backstripping: implications for decompression melting models

The Rattray volcanics lie at the ‘triple junction’ between the three arms of the North Sea rift system. The mildly undersaturated volcanics in this province were erupted during the early syn-rift phase of a Middle-Late Jurassic rifting event, and are thought to be the products of decompression-induced partial melting of the mantle, resulting from lithospheric attenuation during rifting. Situated as they are in one of the most intensively studied rift basins in the world, the Rattray volcanics provide an excellent opportunity to evaluate the relationship between extension and melting described by McKenzie and Bickle [1]. This study uses a modified version of the backstripping technique, which takes into account the flexural strength of the lithosphere, to constrain the stretching factor, β, for the Jurassic rift phase. Two-dimensional interpreted sections are flexurally backstripped to the base post-rift to give an initial estimate of β. β is then further constrained by forward modelling to the base post-rift basin geometry generated by backstripping. Forward modelling uses the flexural cantilever model, a combined simple-shear/pure-shear model for continental extension which takes into account the thermal, rheological and flexural isostatic consequences of extension. This is an approach that is different from that of previous studies [2–5], which assume local, rather than regional, isostatic compensation to allow the backstripping of individual wells. The results obtained in this new study give maximum Jurassic β-factors on each of the three rift arms close to the ‘triple junction’ of 1.2–1.35. This suggests a maximum Jurassic β-factor in the ‘triple junction’ itself of 1.35–1.75. This is considerably lower than previous estimates, and is due to the breakdown of the assumption used in these previous studies that Airy isostasy can approximate the response of a low flexural rigidity lithosphere in subsidence analysis. Whilst such a β-factor is insufficient to generate melt by decompression alone, an anomalously high mantle potential temperature (θ) and the presence of volatiles as well as a complex Mesozoic and Late Palaeozoic rifting history are likely to have played a role in the generation of melt.

Lithospheric necking and regional isostasy at extensional basins 1. Subsidence and gravity modeling with an application to the Gulf of Lions Margin (SE France)

We present a numerical pure‐shear stretching model to study the effect of the “depth of lithospheric necking” on the state of flexure at extensional basins. This model avoids the need for low flexural rigidities during synrift basin subsidence. The model also accounts for long‐standing rift flank uplifts in the absence of significant thermal anomalies or underplating beneath rift shoulders, and it predicts low β factors beneath basin centers. Numerical modeling of synthetic continental rift zones and rifted continental margins together with their gravity characteristics demonstrates that isostatic residual anomalies are sensitive indicators of the state of flexure at extensional basins, particularly at basin margins. Isostatic residual anomalies are particularly useful to study the state of lithospheric flexure because the results are not biased by assumptions on the loads that have to be made in a forward modeling approach. Previous studies of lithospheric flexure and gravity modeling might have underestimated flexural rigidities at extensional basins. Comparison of predicted gravity anomaly signatures with gravity data suggests that at narrow rift basins and young rifted continental margins, upward flexure occurs more commonly than does downward flexure. This can be explained by intermediate or deep levels of necking (&gt;15 km). For rifted continental margins, the apparent transition in the state of flexure during basin evolution from upward to downward flexure points to intermediate levels of necking (15–20 km). The occurrence of intermediate to deep levels of necking at narrow rift basins and intermediate levels of necking at rifted margins is in accordance with brittle‐ductile rheological models of the lithosphere. An intermediate depth range for the level of lithospheric necking at rifted continental margins is consistent with a scenario in which rifting preferentially occurs in an area where the lithosphère is weakened by a thickened crust Rifting in areas of stronger lithosphere, as for example cratons, is probably more often associated with a deep level of necking, explaining the widespread occurrence of rift shoulders at failed rift basins. We have tested several depths of necking and a model of local isostasy for the Gulf of Lions margin in the northwestern Mediterranean. At positions of deep synrift grabens, models with depths of necking of 25–35 km predict crustal thicknesses that are more in accordance with observations than are models with a shallower level of necking. It appears that local isostasy cannot account for the present‐day basin configuration and at the same time reproduce the observed strongly laterally varying bathymetry at the end of rifting, supporting a significant postrift flexural rigidity.

Crustal velocity and Moho topography in central New Hampshire

The White Mountain Batholith in central New Hampshire is a major plutonic, topographic and gravimetric feature, superimposed on the NE‐SW trending regional pattern of accreted terranes in northeastern North America. Compressional and shear‐wave arrival times from a two‐dimensional seismic array experiment are used to constrain a one‐dimensional crustal velocity model and Moho topography for central New Hampshire. Our deployment, a “piggyback” on the Ontario‐New York‐New England refraction experiment (ONYNEX), recorded 10 explosive sources using 20 three‐component digital seismometers. The crust can be modeled as 34.5 km thick, and with three layers having linear gradients without midcrust discontinuities. For this model, P‐velocities are 6.1–6.2 km/s in the upper crust, and 6.2–6.8 km/s in the lower crust. S‐velocities are 3.5–3.7 km/s in the upper crust, and 3.7–3.9 km/s in the lower crust. Because Moho reflections comprise our principal constraints on lower‐crustal velocities, there is a tradeoff between lower‐crustal velocity and crustal thickness. In particular, an average crustal thickness of 39–40 km, reported elsewhere in New England, is not precluded by our data. The Poisson's ratio for the crust varies between 0.23–0.26, in the lower range of values reported for crustal minerals. Our dataset is consistent with ≲2% lateral variation in the upper crust underlying the New Hampshire and White Mountain Plutonic Series, but larger variations deeper than 15 km. If modeled in terms of Moho topography, PmP traveltime residuals and PmP–P residuals suggest thicker crust (36–37 km) under the White Mountain batholith, with thinner crust (33–34 km) to the east and southwest of the White Mountains. Such topography of the Moho correlates well with both the surface elevation and the Bouguer gravity anomaly in the region. Gravity data do not favor the simplest alternative model: a thick low‐velocity granitic lower crust beneath the batholith. Using a simplified density model, roughly 30–35% of the gravity anomaly in the White Mountain region can be modeled as the result of the inferred Moho depression, requiring only a 5 km low‐density root for the White Mountain batholith. The short wavelength of the Moho topography is consistent with a very low flexural rigidity for the lithosphere (≈ 1021 Nt‐m), suggesting that the plutons were intruded into a weak lithosphere. A rapid transition from positive to negative PmP residuals falls along a line that parallels the onshore extension of the Kelvin Seamounts, and may correspond to the “line of weakness” hypothesized by Sykes (1978) to have been tectonically active during the opening of the North Atlantic Ocean at 100–200 Ma.

Evolution of the lithosphere beneath the Michigan Basin

Using depths to formation tops from 480 wells, changes in shape of the Michigan Basin during the Phanerozoic are determined. Flexural rigidity of the lithosphere is then estimated for several time intervals by assuming that changes in shape were caused by deflection of an elastic plate subjected to an axisymmetric load. It is found that the flexural rigidity generally increased over time, from less than 10 22 N m early in the basin's history to more than 10 23 N m late in the basin's development. This increase in rigidity is attributed to cooling and thickening of the elastic portion of the lithosphere as the basin was subsiding. Low flexural rigidity early in the basin's history indicates that the lithosphere was probably rejuvenated (heated, thinned, and possibly uplifted) prior to basin subsidence. The location of the load remained approximately fixed over time. The rejuvenation process is investigated using a one-dimensional finite-difference thermal model in which a 142-km plate is temporarily heated from below. Taking the depth to the 450°C isotherm to be the effective elastic thickness of the lithosphere, rigidity predicted by the thermal model is consistent with the observed increase in rigidity over time. Furthermore, phase changes produced by heating during thermal rejuvenation, combined with the predicted thermal contraction following rejuvenation, explain the two episodes of subsidence observed in the Michigan Basin. We conclude that the lithosphere in this area was rejuvenated approximately 500 m.y. ago. As the lithosphere cooled, it thickened; it also contracted and subsided. Temperatures near the surface of the crust apparently were not raised sufficiently by lithospheric rejuvenation to alter the Keweenawan and older radiometric and paleomagnetic dates of basement rocks.