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Micro-meteoroid and orbital debris (MMOD) refers to millions of micrometeoroids and orbital debris orbiting the Earth, which can come from remaining parts of spacecraft from previous missions. These debris objects keep colliding and creating new pieces of debris, with very different sizes ranging from millimeters to meters. About 99.3% of the debris objects have a size

The Goldstone Solar System Radar (GSSR) facility is the largest fully steerable ground-based radar in the world for nonclassified high-resolution ranging and imaging of planetary and small-body targets. Over the years, the use of the GSSR to detect and characterize near-Earth objects (NEOs) has become critical to keep track of potential Earth-impacting hazardous NEO. This article relates the specific modifications made to the GSSR hardware and procedures in the last few years, as well as the new capabilities derived from those upgrades: reduced complexity in remote operations, increased experimental design versatility, and increased performance on bistatic radar experiments from GSSR to other complexes. In addition, we dedicate a section to provide an update on the current GSSR power capabilities as the new klystrons are installed. The work detailed in this article is intended to reach the broader science community in order to communicate how those modifications and the derived new capabilities can make science experiments more successful.

Using geoid anomalies determined from satellite observations of the South American plate, we demonstrate the existence of a lithospheric flexural forebulge east of the High Andes. Using the planform and location of the geoid anomalies and accounting for the curvature of the Andean orogen, we can successfully model plate flexure using a uniform elastic thickness. Topography above 3 km elevation between −5° and −30° latitude in the Andes loads the margin of the western side of the Precambrian shield of the continental plate and drives bending of the cratonic plate. Removal of horizontal wavelengths greater than 4500 km from the geoid anomaly reveals a 5–7 m positive anomaly paralleling the trend of the orogen some 400 km east of the mountain front. We interpret the secondary geoid high as a flexural forebulge that developed in response to topographic loading of the South American plate by the Andes. While the topographic expression of this forebulge is hidden by the alluvium shed from the Andes and the vegetative cover of the Amazon jungle, our filtered geoid anomalies and a three-dimensional, single-plate flexural model in spherical geometry are both well fit by a single model with ~50 km effective elastic thickness.

A high degree of correlation between present-day relative rock uplift measured using continuous GPS geodesy and spatially averaged surface elevations suggests that long-wavelength topographic relief is presently increasing along the southeastern flank of the Sierra Nevada range and within an adjacent portion of the northern Basin and Range province. Current estimates for erosion rate are an order of magnitude smaller than the relative rates determined by geodesy. Thus, although the uplift serves to enhance long-wavelength relief, it cannot be explained entirely as an isostatic response to erosion. If uplift rates have been constant through time, the duration over which the uplift could have been active (

Lithospheric geoid anomalies record changes in elevation and potential energy experienced by continental lithosphere. Estimates of local isostatic equilibration and potential energy, in tandem with lithosphere-related geoid anomalies, can be used to estimate paleolithospheric thickness, providing a clearer understanding of how and why continental topography is developed. We employ several simplifying assumptions about the crustal and mantle lithosphere density and structure (and readily acknowledge that our results are therefore first-order approximations) to predict the pre-orogenic structure of the lithosphere. At the outset we emphasize that while this approach does not provide an exhaustive evaluation of the deformation mechanism, it does serve to quantify the relative role played by the variations in the crustal and upper mantle components of the lithosphere. In this way we are able to use independent measurement of lithospheric geoid anomalies, current (post-orogenic) elevation and lithospheric structure, and paleoelevation information to estimate topographic development and structural support over time. Application of this technique to the southwestern United States indicates that the uplift of the Colorado Plateau is the result of processes in both the crust and mantle lithosphere and that the lithosphere of the pre-orogenic Southern Basin and Range was thinned relative to the Northern Basin and Range and Colorado Plateau. Although we use the southwestern U.S. as an example, this method can help constrain uplift mechanisms for any region for which the structure and geoid anomaly of the modern lithosphere is well understood.

SUMMARY We present a boundary-layer model for mantle plumes driven by thermal and chemical diffusion and buoyancy. The problem is solved for a Boussinesq, Newtonian fluid with infinite Prandtl number and constant physical properties. We focus on axisymmetric mantle plumes, but also solve 2-D plumes due to line-sources for comparison. The results show that chemical plumes are much thinner than thermal plumes because of small chemical diffusivity in the mantle. When pressure-release partial melting occurs in a thermal-chemical plume, at least two mantle components may be involved: one from the chemical plume and one from the ambient mantle. A buoyant chemical boundary layer in the plume source region tends to cause narrow and strong plumes. A dense chemical source would have the opposite effect. The effects of chemical buoyancy diminish as the Lewis number, the ratio of thermal to chemical diffusivity, increases. For fully developed mantle plumes, the effects of chemical buoyancy may be insignificant. The physical parameters of mantle plumes may be estimated using surface information deduced from swell models. The total heat input from the Hawaiian plume source is about 1.3 x lo1' W, nearly 5-10 per cent of the total heat loss from the core. The depth of the Hawaiian plume source is constrained to be near the core-mantle boundary. Our results show that 2-D plumes are generally stronger than axisymmetric plumes.

Apatite and zircon (U-Th)/He ages from a 100-km-long range-perpendicular transect in the northern Sierra Nevada, California, are used to constrain the exhumation history of the range since ca. 90 Ma. (U-Th)/He ages in apatite decrease from 80 Ma along the low western range flanks to 46 Ma in the higher elevations to the east. (U-Th)/He ages in zircon also show a weak inverse correlation with elevation, decreasing from 91 Ma in the west to 66 Ma in the east. Rocks near the range crest, sampled at elevations of 2200–2500 m, yield the youngest apatite helium ages (46–55 Ma), whereas zircon helium ages are more uniform across the divide. These data reveal relatively rapid cooling rates between ca. 90 and 60 Ma, which are consistent with relatively rapid exhumation rates of 0.2–0.8 km/m.y., followed by a long period of slower exhumation (0.02–0.04 km/m.y.) from the early Paleogene to today. This is reflected in the low-relief morphology of the northern Sierra Nevada, where an Eocene erosional surface has long been identified. A long period of slow exhumation is also consistent with well-documented, widespread lateritic paleosols at the base of Eocene depositional units. Laterites preserved in the northern Sierra Nevada are the product of intense weathering in a subtropical environment and suggest an enduring, soil-mantled topography. We interpret this exhumation history as recording a Late Cretaceous to early Cenozoic period of relatively rapid uplift and unroofing followed by tectonic quiescence and erosional smoothing of Sierran topography through the Neogene. Well-documented recent incision appears to have had little effect on (U-Th)/He ages, suggesting that less than ∼3 km has been eroded from the Sierra Nevada since the early Cenozoic.

Shallow-marine rocks exposed on the 2-km-high, 45-km-thick Colorado Plateau in the western United States indicate that it was near sea level during much of the Phanerozoic. Isostatic calculations, however, illuminate the difficulty in maintaining a 45-km-thick crust at or near sea level. We propose that an isostatically balanced, 30-km-thick, proto‐Colorado Plateau crust was thickened during the Late Cretaceous to early Tertiary by intracrustal flow out of an overthickened Sevier orogenic hinterland. This plateau would have been supported by a thick (>70 km) crustal root, which is proposed to have been the source region for hot and weak mid-crustal material that flowed eastward from the plateau toward the low-elevation proto‐Colorado Plateau.

Bouguer gravity data along the Nazca profile of Fukao et al. (1989), NE-SW striking across the Peruvian Andes, are used to investigate the flexure of the Brazilian Shield. Modeling of the observed gravity data indicates that the Brazilian Shield lithosphere is flexed downward and may extend beneath the Andes as far as 150 km. Assuming that the Brazilian Shield behaves elastically, flexural analysis shows that the isostatic gravity inequilibrium in the eastern Andes can be explained by deflection of the Moho due to the bending of an elastic plate beneath the Sub-Andes. The elastic thickness of the plate is estimated to be between 25 and 55 km, with corresponding flexural rigidity between 0.1 and 1.7 × 10 24 Nm. Many small to moderate-sized earthquakes that have focal depths of tens of kilometers are distributed over a broad area from the Eastern Cordillera to the Brazilian Shield. These events are located well above the subducted flat Nazca plate and very likely are associated with the underthrusting of the Brazilian Shield. A step-like gravity anomaly of 150 mGal on the eastern margin of the Eastern Cordillera cannot be fully explained by the flexure model alone and requires a sharp-edged mass deficit or discontinuous Moho beneath the Eastern Cordillera. Our gravity modeling shows that Bouguer gravity anomalies along the Nazca profile are best explained by a flexed Moho, which is deflected by the underthrusting of the Brazilian Shield beneath the Andes along with a wedge-shaped body of low-density material directly beneath the Eastern Cordillera.

New paleobotanical data suggest that in the late Eocene the erosion surface which capped the Front Range, Colorado was 2.2-2.3 km in elevation, which is similar to the 2.5-km present elevation of surface remnants. This estimated elevation casts doubt on the conventional belief that the low-relief geomorphic surface was formed by lateral planation of streams to a base level not much higher than sea level and that the present deeply incised canyons must represent Neogene uplift of Colorado. Description of the surface, calculations of sediment volume, and isostatic balance and fluvial landsculpting models demonstrate that while the high elevation of the erosion surface was due to tectonic forces, its smoothness was mostly a result of climatic factors. A sediment balance calculated for the Front Range suggests that from 2 to 4 km of material were eroded by the late Eocene, consistent with fission track ages. This amount of erosion would remove a significant portionof the 7 km of Laramide upper crustal thickening. Isostatic modeling implies that the 2.2-3.3 km elevation was most likely created by lower crustal thickening during the Laramide. A numerical model of fluvial erosion and deposition suggests a way that a late Eocene surface could have formed at this high elevation without incision. A humid climate with a preponderance of small storm events will diffusively smooth topography and is a possible mechanism for formation oflow-relief, high-level surfaces. Paleoclimate models suggest a lack of large strom events in the late Eocene because of cool sea surface temperatures in the equatorial region. Return to a drier but stormier climate post-Eocene could have caused the incision of the surface by young canyons. By this interpretation, regional erosion surfaces may represent regional climatic rather than tectonic conditions.

Quantitative models of landform development can help us to understand the evolution of mountains and regional topography, and the effects of tectonic motions and climate on landscape, including its fractal geometry. This paper presents a general and powerful three-dimensional model of fluvial erosion and deposition at hill- to mountain-range scale. The model works by accumulating the effects of randomly seeded storms or floods (precipitons) that cause diffusional smoothing then move downslope on digital topography grids, that erode portions of elevation differences, that transport a slope-limited amount of eroded material, and that deposit alluvium when their sediment-carrying capacity is exceeded. The iteration of these simple and almost linear rules produce very complicated simulated landscapes, demonstrating that complex landscapes do not require complex laws. Each process implemented in the model is affected differently by changes in horizontal scale. Erosion, a scale-free process, roughens topography at all wavelengths. This roughening is balanced by diffusive processes (scaling as 1 L 2 ) at short wavelengths and deposition (scaling (scaling as 1 L 2 ) at long wavelenghts. Such a mixture of scale-free and scale-dependent processes can produce multifractal behavior in the models. The fractal dimension of the model topography is much more sensitive to climatic variables than to tectonic uplift. Landscape evolution may be fractal, but it does not seem to be chaotic. Analysis of topography of areas in southern Arizona using variograms shows approximately fractal behavior, with mean fractal dimension around 2.2–2.3. Departures from an exact fractal relatioship imply that the topography is in detail multifractal. The fractal dimension at short wavelengths is less than that at long wavelengths. This variation could either be caused by the relative strengths of diffusive and erosional processes shaping the topography, or a result of changes in climatic or tectonic conditions still preserved in the landscape.

East-west variation in tectonic activity and strong north-south climatic gradients provide a unique opportunity to study tectonic, climatic and lithologic influences on landscape evolution in the San Gabriel Mountains, California. The competing tendencies of constructive tectonic and degradational climatic effects act against lithologic resistance to influence fluvial systems, and thus the nature of adjacent and nested drainage basins. Landscape fractal dimension (D), a measure of surface roughness over a variety of scales, and the hypsometric integral (I), a measure of the distribution of landmass volume above a reference plane are useful measures of altitudinal variation with scale. As such, they may provide clues as to the relative influences of tectonism, climate and rock type. Topographic analyses of the San Gabriel Mountains clearly indicate that tectonism strongly influences D at range-wavelength scales, while rock-type variation apparently influences D and I at smaller scales. Tectonism is also shown to influence I across the mountain-piedmont junction at all scales investigated. Tectonic activity shows strong negative correlation with both I and D because tectonically active portions of the mountain front do not allow time for much landscape dissection by lower-order streams. Three-dimensional topographic modeling suggests climatic parameters exert a stronger influence on D than does tectonism. This modeling also suggests an inverse correlation between range-scale D and I with varying climate and uplift rate; a positive correlation is observed in the San Gabriel Mountains. We suggest this difference results from (1) differences in uplift style between the San Gabriel Mountains and the models, and/or (2) variation in rock-type erodibility present in the San Gabriel Mountains but which was not modeled. We postulate that the interaction of tectonic, climatic and lithologic parameters influences the stable I and D to which a landscape evolves. Key questions remaining include: (1) the time required to reach a stable I or D after a climatic or tectonic change; (2) what does the fractal nature of topography tell us about the scaling characteristics of major landforming processes; (3) how does climate influence range-scale I and both range- and small-scale D; and (4) what are the effects on I and D of range-scale lithologic variation, both in the models and in real landscapes?

Melt generation and extraction along the Hawaiian volcanic chain should be largely controlled by the thermal structure of the Hawaiian swell and the heat source underneath it. We simulate numerically the time- and space-dependent evolution of Hawaiian volcanism in the framework of thermal evolution of the Hawaiian swell, constrained by residual topography, geoid anomalies, and anomalous heat flow along the Hawaiian volcanic chain. The transient heat transfer problem with melting relationships and variable boundary conditions is solved in cylindrical coordinates using a finite difference method. The model requires the lithosphere to be thinned mechanically by mantle plume flow. Melting starts quickly near the base of the plate when the hotspot is encountered. Thermal perturbation and partial melting are largely concentrated in the region where the original lithosphere is thinned and replaced by the mantle flow. The pre-shield Loihi alkalic and tholeiitic basalts are from similar sources, which are a mixture of at least three mantle components: the mantle plume, asthenosphere, and the lower lithosphere. The degree of partial melting averages 10–20%, with a peak value of 30% near the plume center. As a result of continuous compaction, melts are extracted from an active partial melting zone of about 10–20 km thickness, which moves upwards and laterally as the heating and compaction proceed. The rate of melt extraction from the swell increases rapidly to a maximum value of ∼ 1 × 105 km3/m.y. over the center of the heat source, corresponding to eruption of large amounts of tholeiitic lavas during the shield-building stage. This volume rate is adequate to account for the observed thickness of the Hawaiian volcanic ridge. Melts from direct partial melting of the mantle plume at depth may be important or even dominant at this stage, although the amount is uncertain. At the waning stage, mixing of melts from the mantle flow pattern with those from low-degree partial melting of the lithosphere may produce postshield alkalic basalts. After the plate moves off the heat source, continuous conductive heating can cause very low degree partial melting (less than 1%) of the lithosphere at shallow depths for about one million years. This process may be responsible for producing post-erosional alkalic basalts. The extraction time for removing such small amount of melts is about 0.4–2 m.y., similar to the time gap between the eruption of post-erosional alkalic lavas and the shield-building stage. Our results show that multi-stage Hawaiian volcanism and the general geochemical characteristics of Hawaiian basalts can be explained by a model of plume-plate interaction.

The Ancestral Rocky Mountains (ARM) formed a system of highlands and adjacent basins that developed during Pennsylvanian–earliest Permian deformation of interior western North America. The cause of this intracratonic deformation remains debated, although many have linked it to far-field compression associated with the Carboniferous–Permian Ouachita-Marathon orogeny of southern North America. The ultimate disappearance of the ARM uplifts has long been attributed to erosional beveling presumed to have prevailed into the Triassic–Jurassic. New observations, however, indicate an abrupt and unusual termination for the largest of the ARM uplifts. Field evidence from paleohighlands in the central ARM of Oklahoma and Colorado indicates that Lower Permian strata onlap Pennsylvanian-aged faults and bury as much as 1000 m of relief atop the paleohighlands. In parts of Oklahoma and Colorado, late Cenozoic partial exhumation of these paleohighlands has revealed landscapes dating from Permian time. These relationships suggest cessation of uplift followed by active subsidence of a broad region that encompassed both basins and uplifted crustal blocks and that commenced in Early Permian time, directly following the Pennsylvanian tectonic apogee of the ARM. Independent from these geological observations, geophysical data reveal a regional-scale mafic load underpinning these paleohighlands, emplaced during Cambrian rifting associated with the southern Oklahoma aulacogen. Geophysical modeling of the effects of such a load in the presence of a horizontal stress field, such as that implied by ARM orogenesis, indicates that the amplitude of flexurally supported features is modulated nonlinearly. This leads to buckling and thrust formation with the application of sufficient compressive stress, and subsidence of topography formed by buckling upon relaxation of the high compressional stresses. We therefore infer that the core ARM highlands subsided owing to the presence of a high-density upper crustal root, and that this subsidence began in the Early Permian owing to relaxation of the in-plane compressional stresses that had accompanied the last phase of the Ouachita-Marathon orogeny of southern and southwestern Laurentia. Our results highlight the importance of tectonic inheritance in intraplate orogenesis and epeirogenesis, including its potential role in hastening the reduction of regional elevation, and enabling the ultimate preservation of paleolandscapes.

[Sahagian et al. (2002)][1] provide an interesting look at the uplift history of the Colorado Plateau through an analysis of paleoelevations estimated using a study of basalt vesicularity. They conclude that the Colorado Plateau experienced slow uplift beginning ca. 25 Ma, with rapid uplift since 5

Mantle plumes are often invoked as the ultimate cause of major episodes of continent generation. In this paper we explore the potential of normal plate-tectonic processes to generate intense crustal growth. The central problem is localization of rapid crustal growth into small regions. This can be achieved by transport of terranes parallel to the continental edge in orogenic zones, which we deduce from an analysis of the proportion of present-day continental margins that are dominated by strike-slip motion, together with the proportion of subduction zones showing obliquity >30°. There is a 16% probability of margin-parallel terrane transport on a scale >400 km, and a few margins show transport on a scale >1000 km. The results suggest that concentration of juvenile arc materials into restricted locations can explain both the apparent episodicity and rapid genesis of Precambrian juvenile provinces.

Erosion beveled the Laramide Front Range uplift in Colorado to a surface of low relief by the end of the Eocene. This study uses J.A. Wolfe's new multivariate climate analysis techniques to determine the paleoelevation of this regional surface by examining the overlying 35 Ma Florissant flora. A multiple regression model explaining 93.3% of the variance in mean annual temperature was developed using Wolfe's dataset of 31 leaf physiognomic character states for 86 modern vegetation sites. These character states were scored on 29 species collected from one facies of the Florissant Lake Beds. The paleotemperature estimate of mean annual temperature (10.7 {plus minus} 1.5C) derived from these data, when combined with sea-level temperature and terrestrial lapse rate, implies a late Eocene paleoelevation of 2.4-2.7 km. Pliocene uplift is thus not required to explain the present elevation of 2.5 km. It is unclear when and why the southern Rocky Mountains achieved this elevation. Magmatic crustal thickening can explain the late Eocene high elevation of the southern Rockies, but neither this mechanism nor compressive thickening explains why the Great Plains, which are tied to the Florissant elevation by the Wall Mountain Tuff, were also high. This paleoelevation estimate indicates that regional surfacesmore » of planation could be formed at high elevation in the Eocene, probably because of peculiarities of the Eocene climate.« less

Mineral zoning studies in several granulite terranes have yielded nearly isobaric retrograde pressure-temperature ( P-T ) paths with significant path dispersion. Explanations include magmatic underplating, incomplete isostatic rebound following thrusting, and cooling after tectonic denudation associated with crustal extension. One-dimensional thermal models help to interpret the P-T-t (time) paths generated by such processes. Basaltic underplating yields P-T paths in good agreement with data for the Grenville province of Ontario, but yields T-t paths that disagree strongly with thermochronometry. In addition, seismic, heat-flow, and gravity data from the Grenville province are not consistent with a mafic lower crust. Calculations based on simple thrust models agree with the T-t data, but yield isothermal rather than isobaric initial P-T paths, and consistently underestimate the pressure at which peak temperature is achieved. The only model we found that fits both the P-T and T-t data incorporates a post-thrusting episode of tectonic denudation near the peak of metamorphism. This model also suggests that the temperature dependence of diffusion in garnet may explain the difference between measured isobaric and isothermal paths in granulites and amphibolites.

Writing a review paper about plate tectonics is in a sense like attempting to review the science of geology. This new set of ideas about the large-scale behavior of the outer parts of the earth has had an enormous impact on the thinking of earth scientists in the past few years. The theory of plate tectonics provides unifying concepts that tic together great masses of geological and geophysical information whose relationships were previously obscure. The simplicity and exactness of the theory has made possible successes in quantitative prediction of geological phe­ nomena that were previously only vaguely, if at all, understood. Faced with a large and burgeoning literature concerned with plate tectonics and its applications, we perforce must restrict ourselves to examining a limited selection of the available work. To this end, we have chosen to sketch briefly the underlying theory of plate tectonics and some of the objections that have been raised, then to examine the present and past plate motions in oceanic areas, and finally to discuss some of the implications of plate theory for the geology of the continents. In our selection from the available literature, there are no papers pub­ lished prior to 1960. This reflects merely the tremendous outpouring of recent literature, and we intend no slight to the long and distinguished history of geologic thought on which the conceptual edifice of plate tectonics has been erected.

The geoid is becoming increasingly important in interpretation of global tectonics. Most of the topography of the earth is isostatically compensated, so removal of its effect from the geoid is appropriate before tectonic modeling. The oceanic trenches, however, are dynamically depressed features and cannot be isostatically compensated in the classical way. Continental topography compensated at 35 km gives intracontinental geoidal undulations of up to 15 m over mountain ranges in a spherical harmonic expansion to order and degree 22. Oceanic topography compensated at 40 km, reasonable for the thermally supported long wavelengths, matches the +10 m difference between old continents and old oceans in a detailed NASA/GSFC geoid. Removing the assumed compensation for the oceanic trenches leaves negative anomalies of up to 9 m amplitude caused by their uncompensated mass deficit. This mass deficit acts as a partial "regional compensation" for the excess mass of the subducting slabs, and partly explains why geoidal (and gravity) anomalies over the cold slabs are less than thermal models predict.

Summary. Present-day plate motions imply that about 240 km3 of oceanic lithosphere is created by sea-floor spreading and destroyed by subduction per year. A greater volume of asthenosphere will be dragged along by plate motions. Given the fluxes generated at plate boundaries, the horizontal direction and net rate of counterflow required to maintain mass balance is determined globally by a simple analytical model. Time-dependent calculations indicate that the motions are approximately valid in the hotspot reference frame over the past 5 Myr. Under most plates, the model return flow is opposite to the lithospheric motion in the hotspot frame. The counterflow dominates the resisting stresses to plate motion, so driving force models based on plate drag alone are not valid where the directions of plate motion and counterflow differ. The most marked departure of the two directions is under the North American plate. The model counterflow directions indicate that the sources of mantle hotspots are not located within the asthenosphere. Model flux balances demonstrate exchange of material between asthenospheric reservoirs located beneath different plates. Suggestions of southward asthenospheric motion under the North Atlantic, based on physical features around Iceland and strontium isotope geochemistry, are consistent with the direction of flow predicted by the model.

The non-hydrostatic geoid is dominated by three large anomalies: an area of high gravity potential in the equatorial Pacific; another stretching from Greenland through Africa to the southwest Indian Ocean; and a semi-continuous low region passing from Hudson's Bay through Siberia to India and on to Antarctica. None of these three high-amplitude (greater than 60 m) and long-wavelength anomalies corresponds to present-day plate boundaries. However, if the modern geoid is plotted over the positions of continents and plate boundaries at 125 Ma B.P. (reconstructed relative to hotspots) a strong correlation emerges. The modern geoidal low corresponds in position to the areas of subduction surrounding the Pacific 125 Ma ago. The geoidal high now centered on Africa is entirely contained within ancient Pangaea, and the equatorial Pacific high overlies the location of the spreading centers preserved in the magnetic anomalies of the central Pacific. The most plausible cause of the large geoidal undulations is lower mantle convection only weakly coupled to plate motions. The correspondence between modern geoid and ancient plate boundaries implies either that the coupling was much more intimate in the past, or that there is a lag of at least 100 Ma in response of the lower mantle to upper mantle conditions.

Two segments of the Denali fault system, the McKinley strand west of the Delta River and the Dalton–Shakwak fault east of the Delta River, have nearly perfect small circle geometries. This geometry permits interpretation of the right-lateral slip along these faults in terms of rigid plate tectonics. Their poles of rotation are in the Gulf of Alaska at 59.63°N, 147.38°W and 50.38°N, 154.02°W respectively. A model in which there has been simultaneous motion on both faults since 38 Ma ago predicts a third fault at their juncture which must act as a plate boundary with northwesterly thrust motion in this time interval. The Broxson Gulch thrust, which extends from near the Susitna River to its termination at the Delta River, meets these requirements. Paleozoic and Mesozoic volcanics, as well as Oligocene or younger strata, are thrust beneath sillimanite schists along this fault, and major pre-Tertiary fold structures are truncated by it. Given the direction of tectonic transport on all three faults and a displacement of 38 km on the McKinley strand, the minimum displacements on the Broxson Gulch and the Denali (Dalton–Shakwak) faults in the last 38 Ma are approximately 54 and 90 km respectively. The previously correlated Maclaren and Ruby Range metamorphic belts, however, indicate 300–400 km offset since about 55 Ma ago. Our results require that about 300 km of this be taken up west of the Maclaren belt, either on the McKinley strand or on thrust segments similar to the Broxson Gulch, or both. Our results further indicate that the arcuate shape of these segments of the Denali fault system are intrinsic properties of the faults themselves and that oroclinal bending need not be invoked to explain them.

data are being used to study terrestrial phenomena at scales from tens of thousands of kilometers down to hundreds or even tens of kilometers. This paper focuses on the interpretation of global gravity in the form of the geoid and concentrates on what has already been learned from it about the structure of the Earth at long-to-intermed iate wavelengths. The longest wavelengths of the gravity field are dominated by density distributions that must be supported by very large-scale convective processes. I attempt here to document the claim that these density anomalies are both deep and old. At slightly shorter length scales, the geoidal anomalies give very useful constraints on possible subduction processes and lead to the conclusion that density contrasts in the subducting slabs must be regionally partially compensated and supported from below. At yet shorter length scales, satellite altimetry results give insight into the thermal and mechanical structure of the oceanic litho­ sphere, seeming to favor plate over thermal boundary layer models of cooling. At the shortest length scales, the location and mode of compen­ sation of individual seamounts can be studied.

The Sierra San Pedro Martir of Baja California is the highest of the Peninsular Ranges and has undergone rapid Cenozoic uplift. Recent uplift can be explained as a result of flexural isostasy driven by a previously suppressed crustal root. Two-dimensional modeling of topography and Bouguer gravity data for present and preuplift conditions indicates that a flexural isostatic mechanism for uplift is consistent with the available topographic, geologic, and geophysical data. The crustal root underlying the batholithic rocks of the Sierra San Pedro Martir was probably emplaced during Cretaceous magmatic arc activity. However, existence of a vast, low-relief and low-elevation erosion surface mantled with deposits of fluvial systems originating far to the east suggests that by the early Tertiary the plutons were unroofed, and there was only subtle topographic expression of this crustal thickening. We model the remnant crustal root as regionally compensated (suppressed) beneath a strengthened and continuous elastic plate and topographically expressed as a broad dome. The regional distribution of Tertiary sediments and preserved drainage patterns are geologic evidence of such doming. Since the late Miocene there has been rapid uplift and apparent drainage reversal near the dome apex. The present topography approximates the predicted isostatic response of a doubly broken elastic plate to the previously suppressed root. Several lines of evidence indicate that a major component of uplift of the Sierra San Pedro Martir has occurred during the Pliocene-Quaternary and that the crustal scale faulting that has allowed the range to rise rapidly toward local isostasy is related to full development of the San Andreas-Gulf of California transform system.

The global Nd isotopic evidence for early mantle depletion, which shows that some enriched reservoir must have persisted for at least 300 Ma in the early Archean, is discussed. Numerical models of the isotopic effects of melt extraction from the mantle are used to show that early oceanic crust is a more likely candidate for the enriched reservoir than early continental crust. It is suggested that subduction of an early mafic/ultramafic crust and temporary storage may be responsible for the depletion and high epsilon(Nd) values of the Archean upper mantle.

Geoid models show that net uncompensated masses in subducting lithosphere are less than thermal models predict, and do not require elevation of mantle phase changes in the slabs. The largest deviations from hydrostatic equilibrium ignore plate boundaries and may be ascribed to lower mantle convection uncoupled to plate motions.

Euler vectors (relative angular velocity vectors) have been determined for twelve major plates by global inversion of carefully selected sea-floor spreading rates, transform fault trends, and earthquake slip vectors. The rate information comes from marine magnetic anomalies less than 5 m.y. old, so the motions are valid for post-Miocene times. Plate motions in a mean hotspot frame of reference have also been determined, and statistical confidence limits for all the Euler vectors estimated. Among the consequences of the global motion model is the conclusion that fast-spreading ridges (separation rates greater than 3 cm/yr) have plate motion nearly perpendicular to the strike of the ridge and magnetic anomalies. Four more slowly separating ridges have an average obliquity of spreading of almost 20°. For several plate boundaries, results that differ from previous studies are in agreement with geological evidence. The North and South American plates converge slowly about a pole east of the Antilles and near the Mid-Atlantic Ridge. The results for Africa versus Somalia imply slow east-west extension on the East African Rift Valleys. The pole for motion of Eurasia relative to North America is located near Sakhalin, in accordance with evidence from Siberia and Sakhalin.

Leads in basaltic suites from seven oceanic islands form linear arrays on206Pb/204Pb versus207Pb/204Pb diagrams. These arrays are more reasonably interpreted as secondary isochrons than as mixing lines, because of their systematic relationship. Separate two-stage histories calculated for the leads from each island indicate that the source materials for the magmas were derived from a single primary reservoir with present238U/204Pb of 7.91 ± 0.04 by secondary enrichment in U/Pb at different times from 2.5 to 1 Ga ago. This is confirmed by a plot of isochron slope versus intercept, on which the points describing each island's Pb-Pb array all lie very near a single straight line. The isochrons for the Canary Islands and Hawaii, at least, are significantly different. The208Pb/204Pb versus206Pb/204Pb relationships are less coherent. The lead isotopic characteristics are consistent with a model in which lead in the oceanic island magmas is derived from ancient subducted oceanic crust. In particular, this explains the close relationship between lead in mid-ocean ridge and oceanic island basalts without invoking mixing.

Examination of the shape of the midcontinent gravity high of central North America has led to the hypothesis that the Keweenawan rift system that caused it is the result of plate tectonic interaction. A numerical test has been carried out on the width and postulated transform fault offsets of the gravity high. The exactness of fit to a plate tectonic geometry implies that the continental lithosphere behaved as rigid plates during the Late Precambrian, about 1.1 by ago. This exactness of fit also suggests that the total amount of separation on the Keweenawan rifts is equal to the width of the gravity high. Gravity modelling studies bear out the plausibility of a major amount of rifting, up to 90 km under central Lake Superior. The midcontinent gravity high may represent an intermediate stage of continental rifting, since similar gravity highs and strong associated magnetic anomalies are found on the modern rifted margins of the Atlantic Ocean.

Magnetic anomaly lineations just north of the Phoenix Islands in the western equatorial Pacific trend N 80° E for over 1400 km. They are off-set 180 km left-laterally along an interior fracture zone and truncated to the west by another zone. It is not clear how they are terminated to the east. A Deep Sea Drilling Project hole dates these lineations as about 120 mybp. They represent 20 my or less of history from one flank of an early Cretaceous spreading system.

The Juan de Fuca, Rivera, Cocos and Nazca plates in the eastern Pacific are small fragments remaining from the fragmentation and partial subduction of the long, narrow Farallon Plate which began as early as 55 Myr ago1. This fragmentation apparently resulted from interaction of the very irregular Pacific–Farallon ridge with the uneven coastlines of the American Plate1–4 and involved the formation of ridge–trench transforms5, creation of new spreading centres6, ridge jumps1,7,8, deactivation of subduction zones2 and other phenomena1. We present here magnetic evidence that a fifth small fragment of the Farallon plate is preserved in the eastern Pacific off the coast of Baja, California. Its preservation is the result of an eastward jump of the Pacific–Farallon ridge ∼12.5 Myr ago which was preceded by asymmetric spreading and perhaps pivotal subduction1. Menard1 first suggested this idea and showed that anomaly 5B is symmetric and parallel to a linear trough trending southwards from Guadalupe Island. He named the small fragment of the Farallon plate preserved between this trough and the margin of Baja California to the east, the Guadalupe plate1. We show that both anomalies 5B and 5C symmetrically flank the Guadalupe trough. Our results and conclusions are not based on new magnetic data, but rather on reinterpretation of aeromagnetic profiles published by Taylor et al.9.

During the Messinian Stage (5.5 Myr, Miocene/Pliocene boundary) the 4.2×1023 m3 of water that now fills the Mediterranean evaporated. Evidence for this includes palaeogorges 1 km below the present Nile and Rhone valleys and evaporite deposits, sampled by cores and deep-sea drilling1,2, that are thicker than 1 km over much of the Mediterranean3. Two-dimensional flexure models, presented here, indicate that the regionally compensated crustal upwarping from removal of the seawater load would lead to a Messinian geomorphology with an uplifted Mediterranean basin and shoreline bulges along its northwestern and southeastern coasts. These shoreline bulges would cause a reversal of downhill gradient direction in areas with low original seaward slopes. Such a profile would lead to a landward reversal of drainage in rivers with low discharge.

Gravity modeling shows that the observed fluctuations in gravity over the Catalina-Rincon metamorphic core complex of southeastern Arizona are caused primarily by shallow crustal density contrasts. The Wilderness suite granites, the probable source of a 20-mgal low over the central part of the core complex, may extend to depths of 7–12 km below the surface. Seismic refraction information is consistent with the existence of a deep crustal root under the Catalina-Rincon core complex. We hypothesize that the voluminous Wilderness suite granites are related to a crustal thickening event that built the root in early Tertiary time. Modeling of flexural isostatic uplift, resulting from an excess crustal root, predicts two episodes of uplift for the Catalina-Rincon metamorphic core complex: (1) low-relief flexural uplift in the middle Tertiary and (2) complete uplift precipitated by high-angle Basin and Range faulting at 10–15 Ma. The uplift and exposure of core complexes, therefore, can be an isostatically driven process that occurs in regions of locally thickened crust.

Two-dimensional gravity and flexural models constrain the structure and isostatic compensation of the Wind River Range and Granite Mountains, Wyoming. These ranges are asymmetric, basement-cored Laramide uplifts. Their large positive isostatic gravity anomalies imply that the topography of the uplifts is not locally compensated. For the Wind River Range, gravity data are consistent with thrusting along a listric fault which flattens and soles out at 30 km depth with no detectable offset on the Moho. We model the large Bouguer gravity high over the range with high-density lower crustal rocks emplaced over lower-density middle crustal rocks along the Wind River thrust. The high positive gravity anomaly over the Granite Mountains can be modeled as an extension of the South Pass greenstone belt from the southern Wind River Range eastward beneath the Emigrant Trail thrust. A crustal root, as required in local (Airy) compensation, for either range requires unreasonably high-density material in the upper crust to obtain the observed anomaly. Combined gravity and flexural models show that a flexural rigidity at least 2.55×1022 N m (maximum deflection of 2 km of the Moho) is required to satisfy the gravity data. Our results are consistent with models of uplift due to horizontal compression and support of the resulting topography by lithospheric strength. An otherwise puzzling episode of Oligocene uplift of the Wind River Range can be explained as “unbuckling” (decrease of deflection of lithosphere) accompanying release of Laramide compressional stresses. Flexurally supported uplifts will collapse if the lithosphere is weakened. A number of Laramide uplifts have collapsed, primarily east-west trending structures such as the Granite Mountains, the southern end of the Wind River Range, and the Uinta Mountains. We propose that collapse has occurred along zones of established weakness in the crust in response to north-south extension.

The Sierra Nevada of California displays an isostatic gravity anomaly of +35 mGal over the western foothills and −45 mGal at the crest of the range. A flexural isostatic model designed to explain the rapid Neogene uplift of the Sierra Nevada is consistent with these observations and can explain a portion of both gravity anomalies by contributions from Moho depths. The present-day crustal root causing regional compensation in this model differs from the simple local Airy compensation assumed for the Sierra in calculating the isostatic residual gravity. Two-dimensional gravity modeling indicates that this difference results in a residual gravity high of +9 mGals over the western foothills, which are buoyed up by the excess root to the east, and a low of −16 mGals over the crest of the Sierra. The remainder of the isostatic gravity anomaly is attributed to upper crustal density contrasts because high-density rocks underlie the foothills, and a low-density batholith forms the crest. Any model for the isostatic residual gravity must assume, implicitly or explicitly, a flexural response of the lithosphere. With the stiff Sierran lithosphere, the isostatic deflection of the Moho due to the upper crustal bodies will be small. If a similar gravity model were to assign no stiffness to the lithosphere (local Airy compensation), the mass anomalies of the upper crustal bodies would necessitate a change in depth of the Moho to achieve compensation. This deflection of the Moho would significantly reduce the amplitude of the anomaly produced by the upper crustal block. For the unbroken, pre-uplift case of the Sierra Nevada of 10 Ma, our model predicts an isostatic gravity low over the range crest of approximately −80 mGals and a flanking high of +55 mGals. These values are larger than would be predicted for a more typical eroded magmatic arc that does not have abnormally low heat flow and consequent high flexural rigidity.

The mechanics of low-angle normal faulting has proved difficult to establish despite clear geological evidence for the existence of such faults. Gently dipping shear stress trajectories are necessary for nucleation and propagation of low-angle faults in a structurally isotropic medium. Such trajectories are not produced by simple horizontal extension, but do arise in the presence of flexural stresses. We model stress conditions in a cross section through the strong upper crust using Airy stress functions for a two-dimensional elastic medium. Numerical modeling indicates that high ratios of flexural stress to extensional stress produce conditions favorable for low-angle normal fault initiation at intermediate depths in an elastic medium. We propose that the gentle dip of Cenozoic low-angle normal faults in the Basin and Range Province resulted from flexural stresses that were produced by isostatically uncompensated surface and, especially, Moho relief. Resurgent mid-Cenozoic magmatism and the consequent reduction of the thermally dependent flexural strength of the mantle lithosphere transferred flexural stresses to the upper crust, causing or promoting initiation of low-angle normal faults. Temporal changes in structural style, from dominantly low-angle to dominantly high-angle normal faulting, may reflect reduction of the magnitude of flexural stresses in the upper crust due to flattening of the Moho and cooling of the mantle lithosphere.

The evolution of midplate hotspot swells on an oceanic plate moving over a hot, upwelling mantle plume is numerically simulated. The plume supplies a Gaussian-shaped thermal perturbation and thermally-induced dynamic support. The lithosphere is treated as a thermal boundary layer with a strongly temperature-dependent viscosity. The two fundamental mechanisms of transferring heat, conduction and convection, during the interaction of the lithosphere with the mantle plume are considered. The transient heat transfer equations, with boundary conditions varying in both time and space, are solved in cylindrical coordinates using the finite difference ADI (alternating direction implicit) method on a 100 x 100 grid. The topography, geoid anomaly, and heat flow anomaly of the Hawaiian swell and the Bermuda rise are used to constrain the models. Results confirm the conclusion of previous works that the Hawaiian swell can not be explained by conductive heating alone, even if extremely high thermal perturbation is allowed. On the other hand, the model of convective thinning predicts successfully the topography, geoid anomaly, and the heat flow anomaly around the Hawaiian islands, as well as the changes in the topography and anomalous heat flow along the Hawaiian volcanic chain.

Gravity and most seismic interpretations agree that the Sierra Nevada is at present isostatically compensated by a crustal root. The most reasonable timing for emplacement of the root was during Mesozoic batholithic intrusions. This is difficult to reconcile with the evidence that major uplift of the mountains occurred in the past 10 m.y. A simple quantitative model for flexural isostasy of an elastic plate before and after breaking resolves this problem and explains the tilt of the Sierra Nevada block and variations in topography along the range. We postulate that cooling and elastic strengthening of the Mesozoic magmatic arc prevented its reaching local isostatic equilibrium during erosion in the Cenozoic. Thus, an overcompensated residual mountain range was held down elastically until Basin and Range extension broke the elastic plate along the Owens Valley and allowed rapid uplift. The Moho density contrast that best models the amplitude of both late Cenozoic uplift and prior topography is slightly larger than seismic and gravity models suggest; this implies that up to 20% of the uplift may be driven by density contrasts in the upper mantle. The idea of overcompensated erosional remnants and their release upon lithospheric faulting has a more general application to “anorogenic” uplift of mountain blocks.

The Sierra Nevada of California has experienced several kilometers of uplift in the last 10 m.y., but its present elevation is isostatically compensated by a crustal root that has probably existed since Late Cretaceous. Flexural isostasy can reconcile these observations: Basin and Range faulting and extension starting 10 m.y. ago allowed rapid uplift of an eroded and regionally compensated magmatic arc underlain by an excess crustal root. A quantitative model of an unbroken elastic plate buoyed up by the excess root provides an explanation for the estimated 10-m.y. paleotopography of the range. The same model fits the present topography when broken (shear and fiber stresses not transmitted) at the location of the Owens Valley fault zone. The broken-plate model predicts the appropriate amount of westward tilt of the Sierra Nevada and can explain the pronounced eastward tilt of the White and Inyo mountains. The position where paleo and present drainages in the Sierra cross over in elevation is the most sensitive indicator of the elastic thickness required of the lithosphere. An effective elastic thickness of 50 km best fits the observed crossover distances and is consistent with the amount of post-10 m.y. uplift. Calculated fiber and shear stresses suggest that actual extension of the crust is more important in permitting the uplift than the mere presence of normal faults. Part of the residual isostatic gravity anomaly pattern (high gravity over the eastern Great Valley and western Sierra Nevada, low residual gravity over the high Sierra) may be explained by the continuing flexural support of departure from local isostatic equilibrium. The model implies that before 10 m.y. ago the Sierra should have been marked by a negative isostatic gravity anomaly of more than −100 m Gal, flanked by isostatic gravity highs. Such large isostatic anomalies have not been observed to date over eroded arcs, but the Sierra has uniquely low heat flow and, presumably therefore, high lithospheric strength, and the Sierran root is at the upper limit of size that could be isostatically suppressed.

The spatial distribution and episodic nature of behind-arc spreading is as yet unexplained. The present sites of rapid marginal basin opening well-developed enough to be producing unmistakable oceanic crust are the Tonga-Kermadec, Mariana, and Scotia arcs and the Andaman Sea. In relation to a hot spot reference frame the major plate lying behind the trench in all these spreading arc systems moves away from the subduction zone (retreats). Taking into account the behind-arc spreading, almost all of the oceanic subduction zones advance in the hot spot frame, i.e., move toward the side from which the subducted slab approaches. This leads to the suggestion that marginal basin opening may be related to a component of ‘trench suction’ in the driving forces of plate tectonics.

A set of east-trending magnetic anomalies located in the western equatorial Pacific Ocean near the Phoenix Islands is Early Cretaceous in age. The use of magnetic reversal model studies shows that this lineated anomaly pattern correlates with one east of Japan that trends east, and with one west of Hawaii that trends northwest. These patterns were formed in their present relative positions, but about 40° (4,500 km) south of their present geographic locations. The configuration of these three contemporaneous sets of magnetic anomalies implies that the Late Mesozoic tectonic pattern consisted of five spreading centers joined at two triple points. In this interpretation, the oldest part of the Pacific Ocean lies just east of the Mariana Trench and is Early Jurassic in age. This Mesozoic system evolved into the Cenozoic spreading pattern recorded in the eastern Pacific Ocean. The details of this transition are open to speculation because it occurred during a period in the Late Cretaceous that lacked magnetic reversals. We propose a model that suggests the northern triple point jumped southeast about 2,000 km at 100 m.y. B.P., and that the Emperor Trough was a transform fault of large offset during the Late Cretaceous. The southern triple point migrated rapidly toward the south-southeast, approximately parallel to the Eltanin Fracture Zone–Louisville Ridge complex that we extend o t the westernmost of the Phoenix lineation fracture zones.

The Lau Basin, which lies between the Tonga and Lau Ridges, is characterized by an absence of sediment, high but variable heat flow, and a confused magnetic anomaly pattern. A ridge 300 km long and 40 km wide runs northwest-southeast through the western part of the basin. This ridge is composed of fresh tholeiitic basalt and is associated with a linear band of shallow-focus earthquakes ( The geophysical and geological data are interpreted within the framework of the theory of plate tectonics. It is suggested that the ridge and related earthquake epicenters mark the boundary between the India plate and a much smaller plate, the Tonga plate, which lies between the India plate and the Tonga Trench. The boundary is a transform fault marking the direction of motion of the two plates. A revised crustal consumption rate of 11 cm/yr for the Tonga Trench is required if the Tonga-India plate separation is added to the India-Pacific plate convergence. Continued NW-SE dilation and basalt intrusion during the past 10 m.y. has separated the Tonga and Lau Ridges and can explain the fresh basalt, the absence of sediment, and the high heat flow in the Lau Basin. This basin, although lying below the ridges on either side, has an average elevation more than 3,000 m above that of the deep Pacific. Intrusion of hot material at the center of the basin can account for this increase in elevation. The classic concept of the “andesite line,” separating oceanic and continental crust, clearly has no meaning as an indication of crustal type, as oceanic crust can be generated behind island arcs.