Thickness Of Continental Lithosphere Research Articles

Constraining Earth’s nonlinear mantle viscosity using plate-boundary resolving global inversions

Variable viscosity in Earth's mantle exerts a fundamental control on mantle convection and plate tectonics, yet rigorously constraining the underlying parameters has remained a challenge. Inverse methods have not been sufficiently robust to handle the severe viscosity gradients and nonlinearities (arising from dislocation creep and plastic failure) while simultaneously resolving the megathrust and bending slabs globally. Using global plate motions as constraints, we overcome these challenges by combining a scalable nonlinear Stokes solver that resolves the key tectonic features with an adjoint-based Bayesian approach. Assuming plate cooling, variations in the thickness of continental lithosphere, slabs, and broad scale lower mantle structure as well as a constant grain size through the bulk of the upper mantle, a good fit to global plate motions is found with a nonlinear upper mantle stress exponent of 2.43 [Formula: see text] 0.25 (mean [Formula: see text] SD). A relatively low yield stress of 151 [Formula: see text] 19 MPa is required for slabs to bend during subduction and transmit a slab pull that generates asymmetrical subduction. The recovered long-term strength of megathrusts (plate interfaces) varies between different subduction zones, with South America having a larger strength and Vanuatu and Central America having lower values with important implications for the stresses driving megathrust earthquakes.

A Newly Discovered Late‐Cretaceous East Asian Flat Slab Explains Its Unique Lithospheric Structure and Tectonics

AbstractThe existence of historical flat slabs remains debated. We evaluate past subduction since 200 Ma using global models with data assimilation. By reproducing major Mesozoic slabs whose dip angles satisfy geological constraints, the model suggests a previously unrecognized continental‐scale flat slab during the Late Cretaceous beneath East Asia, a result independent of plate reconstructions, continental lithospheric thickness, convergence rate, and seafloor age. Tests show that the pre‐Cretaceous subduction history, both along the western Pacific and Tethyan trenches, is the most important reason for the formation of this prominent flat Izanagi slab. Physically, continuing subduction increases the gravitational torque, which, through balancing the suction torque, progressively reduces dynamic pressure above the slab and decreases the slab dip angle. The flat Izanagi slab explains the observed East Asian lithospheric thinning that led to the formation of the North‐South Gravity Lineament, tectonic inversion of sedimentary basins, uplift of the Greater Xing'an‐Taihang‐Xuefeng mountains and the abrupt termination of intraplate volcanism during the Late Cretaceous.

Causes and consequences of asymmetric lateral plume flow during South Atlantic rifting

Volcanic rifted margins are typically associated with a thick magmatic layer of seaward dipping reflectors and anomalous regional uplift. This is conventionally interpreted as due to melting of an arriving mantle plume head at the onset of rifting. However, seaward dipping reflectors and uplift are sometimes asymmetrically distributed with respect to the subsequent plume track. Here we investigate if these asymmetries are induced by preexisting lateral variations in the thickness of continental lithosphere and/or lithospheric stretching rates, variations that promote lateral sublithospheric flow of plume material below only one arm of the extending rift. Using three-dimensional numerical experiments, we find that South Atlantic rifting is predicted to develop a strong southward asymmetry in its distribution of seaward dipping reflectors and associated anomalous relief with respect to the Tristan Plume that "drove" this volcanic rifted margin, and that the region where plume material drains into the rift should experience long-lived uplift during rifting-both as observed. We conclude that a mantle plume is still needed to source the anomalously hot sublithospheric material that generates a volcanic rifted margin, but lateral along-rift flow from this plume, not a broad starting plume head, is what controls when and where a volcanic rifted margin will form.

The Thermal Model of Continental Lithosphere Thickness Changes over Time

The Thermal Model of Continental Lithosphere Thickness Changes over Time

Supercomputer simulation of continental collisions in Precambrian: The effect of lithosphere thickness

Many aspects of Precambrian tectonics are still unclear due to the undetermined influence of several key physical parameters (the temperature of the mantle, the thickness of the lithosphere, etc.) on the geodynamic processes. The values of these parameters in the Precambrian are known to have been significantly different from current conditions. This work presents the results of two-dimensional (2D) numerical petrological-thermomechanical experiments that simulate the process of plate convergence at a velocity of 5 cm/year depending on the thickness of the continental lithosphere. The model continental-lithosphere thickness ranged from 100 to 200 km, the modern mantle temperature exceeded 150°C, while radiogenic heat generation in the continental crust was 1.5 times higher then that at present. The numerical simulations showed that if the lithosphere is 100–160 km thick, the subduction process (closure of an ocean) is terminated by detachment of the oceanic plate from the continental plate (slab break-off) followed by the formation of a large igneous province (an oceanic plateau) between the continents instead of orogeny. The thinner the lithosphere is, the earlier and closer to the surface the slab break-off occurs. Thus, for a model with a continental lithosphere thickness of 150 km, the slab was detached in 10.3 Ma at a depth of 150 km, whereas for a lithosphere of 100 km thick it occurred in just 5.1 Ma almost at the surface. In the latter case, the magma generation area becomes much larger due to the formation of igneous provinces on both sides of the oceanic slab instead of one side, as is proposed in other models. Collision of continents with a very thick lithosphere (200 km or more) is not accompanied by slab break-off and significant volcanic activity. Thus, the results of our modeling show that the SLAB PULL mechanism in a subduction zone contributes to the processes at a plate convergent boundary.

Velocity structure beneath the southern Puna plateau: Evidence for delamination

The high elevation of the southern Puna plateau, the widespread melting of its crust, the gap in intermediate depth seismicity and the recent eruptions of ignimbrite complexes can be explained by delamination of the lithospheric mantle beneath it. To test this hypothesis, an array consisting of 73 broad band and short period seismic stations was deployed in the region for a period of 2 years starting in 2007. We inverted the data using the two plane wave approach and obtained 1‐D and 3‐D Rayleigh wave phase velocities. Our dispersion curve shows that at short periods (<70 s) the phase velocities are slightly higher than those of the Tibetan plateau and lower than those of the Anatolian plateau. At periods of 100–140 s we observe a low velocity zone that might be remnant hot asthenosphere below a flat slab (7–10 Ma). We estimate the average continental lithosphere thickness for the region to be between 100 and 130 km. Our three‐dimensional Rayleigh wave phase velocities show a high velocity anomaly at low frequencies (0.007, 0.008, and 0.009 Hz) slightly to the north of Cerro Galan. This would be consistent with the hypothesis of delamination in which a piece of lithosphere has detached and caused upwelling of hot asthenosphere, which in turn caused widespread alkaline‐collision related volcanism. This interpretation is also corroborated by our shear wave velocity model, where a high velocity anomaly beneath the northern edge of Cerro Galan at 130 km depth is interpreted as the delaminated block on top of the subducting Nazca slab.

Insights into the evolution of the Italian lithospheric structure from S receiver function analysis

To advance our understanding of the complex and episodic tectonic history of the central Mediterranean, we image the characteristics and structure of the convergent African and Eurasian lithospheric plates using S receiver functions. Specifically we investigate the lithospheric structure in terms of seismic velocity discontinuities, including the crust–mantle boundary (Moho) and the lithosphere–asthenosphere boundary (LAB) beneath Italy. The geometry and continuity of these structures are explored using teleseismic S-to-P converted phases, applied extensively throughout this region for the first time. The continental lithospheric thickness varies between ∼60 and ∼170km with an average of ∼90km, and the pulse characteristics of the converted phases can be characterized by tectonic region and history. Beneath the Northern Apennines, where subduction is still active, we find evidence for a complex lithospheric structure, where two different negative S-velocity jumps are present at approximately 90 and 180km depth. The shallowest negative phases are interpreted as the base of the lithosphere, but the second negative phases are much deeper than expected for the overriding plate lithosphere. When compared to hypocenter locations and P-wave tomography these deep converted phases correlate with the base of fast velocities perturbations in the upper mantle and regions with subcrustal earthquakes. In contrast beneath the Southern Apennines, the lithospheric structure appears less complex, with only one significant negative arrival in the receiver functions and where no subcrustal earthquakes are present. These pulses are generally deeper (∼110km) than those beneath the Northern Apennines and are broader in shape indicating a less sharp velocity contrast. Farther south along the Italian peninsula, the structure becomes complex again, with two sharp negative amplitude phases at depth. These deep converted phases that originate in the uppermost mantle beneath the Calabrian arc and Sicily are also co-located with deep (>35km) earthquakes. These observations suggest that these deepest negative phases correspond to the base of the actively subducting lithosphere beneath the Northern Apennines and Calabria, and where these phases are absent in the receiver functions there is no subducting slab. Seismic imaging of the lithosphere and uppermost mantle using S-to-P conversions provides another geophysical dataset to aid in interpretation of the complex, segmented slab structure beneath Italy.

Constraints on upper mantle viscosity from the flow‐induced pressure gradient across the Australian continental keel

The thickness of continental lithosphere varies considerably from tectonically active to cratonic regions, where it can be as thick as 250–300 km. Embedded in the upper mantle like a ship, when driven to move by a velocity imposed at the surface, a continental keel is expected to induce a pressure gradient in the mantle. We hypothesize that the viscosity of the asthenosphere or the shear coupling between lower lithosphere and asthenosphere should control this pressure effect and thus the resulting dynamic topography. We perform three‐dimensional finite element calculations to examine the effects of forcing a continental keel by an imposed surface velocity, with the Australian region as a case study. When the upper mantle is strong but still weaker than the lower mantle, positive dynamic topography is created around the leading edge, and negative dynamic topography is created around the trailing edge of the keel, which is measurable by positive and negative geoid anomalies, respectively. For a weak upper mantle the effect is much reduced. We analyze geoidal and gravity anomalies in the Australian region by spatiospectral localization using Slepian functions. The method allows us to remove a best fit estimate of the geographically localized low spherical harmonic degree contributions. Regional geoid anomalies thus filtered are on the order of ±10 m across the Australian continent, with a spatial pattern similar to that predicted by the models. The comparison of modeled and observed geoid anomalies places constraints on mantle viscosity structure. Models with a two‐layer mantle cannot sufficiently constrain the ratio of viscosity between the upper and lower mantle. The addition of a third, weak, upper mantle layer, an asthenosphere, amplifies the effects of keels. Our three‐layer models, with lower mantle viscosity of 3 × 1022 Pa s, suggest that the upper mantle (asthenosphere) is 300 times weaker than the lower mantle, while the transition zone (400–670 km depths) has a viscosity varying between 1021 and 1022 Pa s.

The fate of the Juan de Fuca plate: Implications for a Yellowstone plume head

Beneath the Pacific Northwest the Juan de Fuca plate, a remnant of the Farallon plate, continues subducting beneath the North American continent. To the east of the Cascadia subduction zone lies the Yellowstone Hotspot Track. The origins of this track can be traced back to the voluminous basaltic outpourings in the Columbia Plateau around 17 Ma. If these basalts are the result of a large melting anomaly rising through the mantle to the base of the North American continent, such as a mantle plume head, the anomaly would need to punch through the subducting Juan de Fuca slab. Here, we use teleseismic body-wave travel-time tomography to investigate the fate of the subducted slab and its possible interaction with a plume head. Our dataset is derived from the Oregon Array for Teleseismic Study (OATS) deployment in Oregon and all other available seismic data in this region during the same period. In our JdF07 models, we image the subducted Juan de Fuca plate in the mantle east of the Cascades beneath Oregon, where the slab has not been imaged before, to a depth of 400 km but no deeper. The slab dips ∼ 50°E and has a thickness of ∼ 75 km. Immediately beneath the slab, we image a low velocity layer with a similar geometry to the slab and extending down to at least ∼ 575 km depth in the V s model. The total length of the high velocity slab is ∼ 660 km, about 180 km longer than the estimated length of slab subducted since 17 Ma. Assuming similar slab geometry to today, this 180 km length of slab would reach ∼ 60 km depth, comparable to the thickness of continental lithosphere. We propose that the absence of the slab below 400 km today is due to the arrival of the Yellowstone plume head ∼ 17 Ma, which destroyed the Juan de Fuca slab at depths greater than the thickness of the continental lithosphere. Given this scenario, the low velocity anomaly beneath the slab is likely the remnant plume head material which has been pulled down by traction with the subducting plate. The amplitude of the observed low velocity anomaly is comparable with that expected for plume head material 100–300 °C hotter than the surrounding asthenosphere.

Convection under a lid of finite conductivity: Heat flux scaling and application to continents

A scaling law for the heat flux out of a convective fluid covered totally or partially by a finitely conducting lid is proposed. This scaling is constructed in order to quantify the heat transfer out of the Earth's mantle, taking into account the effect of the dichotomy between oceans and continents, which imposes heterogeneous thermal boundary conditions at the surface of the mantle. The effect of these heterogeneous boundary conditions is studied here using simple two‐dimensional models, with the mantle represented by an isoviscous fluid heated from below and continents represented by nondeformable lids of finite thermal conductivity set above the surface of the model. We use free‐slip boundary conditions under the oceanic and continental zones in order to study in an isolated way the possible thermal effect of continents, independently of all mechanical effect. A systematic study of the heat transfer as a function of the Rayleigh number of the fluid, of the width of the lid, and of its thermal properties is carried out. We show that estimates of continental lithosphere thickness imply a strong insulating effect from continents on mantle heat loss, at least locally. The heat flux below continents was low in the past and of the order of the present one if the continental thickness has remained broadly constant over the Earth's history.

Formation of continental flood volcanism — The perspective of setting of melting

Models of continental flood basalt (CFB) formation are evaluated by examining their implications for the setting, mainly temperature and depth, of melting which is assumed to result from adiabatic decompression. Most attractive is the model of melting in upwelling bodies (probably plume heads) rising to the base of the continental lithosphere. This constrains the melting to 120–150 km or deeper (continental lithospheric thickness) and thus the plume potential temperatures to ≥ 300 °C higher than ambient mantle. The primary melts should be hot, MgO-rich, modified during ascent, and assimilate components of the lithosphere, which can provide the continental-like geochemical signature of many CFB. Circulation within the upwellings and presence of eclogite patches also influence magma generation and composition. Dehydration melting when plumes heat the lowermost lithosphere can generate CFB only if the source region contains ca. 15% hydrous minerals beneath the entire area covered by flood volcanics, which is difficult to justify. On the other hand, assimilation of “continental” chemical components from large parts of the lithosphere does not require such extreme metasomatism. Decompression melting under strongly thinned rifted lithosphere requires lower potential temperatures of the rising material and lesser modification of the primary magmas than the plume head model of CFB formation. Available observations do not support the contemporaneity of flood volcanism with rifts having the required sizes and histories, but more information is needed to further test this model. On the other hand, magma production can assist rift initiation and lithospheric rupture, so subsequent thinning can explain the common formation of volcanic rifted margins immediately following CFB emplacement. Ancient LIP should record the same processes as seen in young CFB.

Controls of stable continental lithospheric thickness: the role of basal drag

The lithosphere is the earth's strong outer layer that remains a coherent unit for long times during plate motions. The motions lead to shearing of its hot and ductile lower part. The base of the lithosphere is placed where the shearing is large enough to effectively decouple the coherent plates from the underlying mantle. Here the depth and temperature where this occurs under stable continental areas were modeled by using the experimentally determined creep properties of olivine (which depend on temperature, pressure and fluid content) for different geotherms and different magnitudes of the basal drag. It is found that the shearing increases rapidly downward in a ca. 20–25 km thick decoupling zone where the bottom of the lithospherecan can be placed. This zone is deeper for cooler geotherms and for smaller a basal drag, while the presence of fluids makes it shallower. In dry rock this zone is at the mantle adiabat or cooler when the basal drag is > 0.5 MPa. Under Archean cratons whose temperature is represented by xenolith geotherms the decoupling zone is 180–240 km deep. Higher crustal heat generation than in cratons (common in post-Archean terranes) by itself causes the decoupling zone to be a few tens of kilometers shallower, and it will be even shallower if the mantle heat flow is larger than under cratons. The base of the lithosphere need not follow an isotherm. As the lithospheric thickness depends on several factors it will change when any of these changes, which is expected to have happened many times in earth history. To persist the lithosphere should escape being sheared off laterally, regardless of its buoyancy.

Thermal thickness and evolution of Precambrian lithosphere: A global study

The thermal thickness of Precambrian lithosphere is modeled and compared with estimates from seismic tomography and xenolith data. We use the steady state thermal conductivity equation with the same geothermal constraints for all of the Precambrian cratons (except Antarctica) to calculate the temperature distribution in the stable continental lithosphere. The modeling is based on the global compilation of heat flow data by Pollack et al. [1993] and more recent data. The depth distribution of heat‐producing elements is estimated using regional models for ∼300 blocks with sizes varying from 1°×1° to about 5°×5° in latitude and longitude and is constrained by laboratory, seismic and petrologic data and, where applicable, empirical heat flow/heat production relationships. Maps of the lateral temperature distribution at depths 50, 100, and 150 km are presented for all continents except Antarctica. The thermal thickness of the lithosphere is calculated assuming a conductive layer overlying the mantle with an adiabat of 1300°C. The Archean and early Proterozoic lithosphere is found to have two typical thicknesses, 200–220 km and 300–350 km. In general, thin (∼220 km) roots are found for Archean and early Proterozoic cratons in the Southern Hemisphere (South Africa, Western Australia, South America, and India) and thicker (>300 km) roots are found in the Northern Hemisphere (Baltic Shield, Siberian Platform, West Africa, and possibly the Canadian Shield). We find that the thickness of continental lithosphere generally decreases with age from >200 km beneath Archean cratons to intermediate values of 200±50 km in early Proterozoic lithosphere, to about 140±50 km in middle and late Proterozoic cratons. Using known crustal thickness, our calculated geotherms, and assuming that isostatic balance is achieved at the base of the lithosphere, we find that Archean and early Proterozoic mantle lithosphere is 1.5% less dense (chemically depleted) than the underlying asthenosphere, while middle and late Proterozoic subcrustal lithosphere should be depleted by ∼0.6–0.7%. Our results suggest three contrasting stages of lithosphere formation at the following ages: >2.5 Ga, 2.5–1.8 Ga, and <1.8 Ga. Ages of komatiites, greenstone belts, and giant dike swarms broadly define similar stages and apparently reflect secular changes in mantle temperature and, possibly, convection patterns.

Young mafic back arc volcanic rocks as indicators of continental lithospheric delamination beneath the Argentine Puna Plateau, central Andes

The spatial distribution of some major and trace element and isotopic characteristics of backarc Plio‐Quaternary basaltic to high‐Mg andesitic (51% to 58% SiO2) lavas in the southern Puna (24°S to 27°S) of the Central Andean Volcanic Zone (CVZ) reflect varying continental lithospheric thickness and the thermal state of the underlying mantle wedge and subducting plate. These lavas erupted from small cones and fissures associated with faults related to a change in the regional stress system in the southern Puna at ≈ 2 to 3 Ma. Three geochemical groups are recognized: (1) a relatively high volume intraplate group (high K; La/Ta ratio <25) that occurs over a thin continental lithosphere above a gap in the modern seismic zone and represents the highest percentage of mantle partial melt, (2) an intermediate volume, high‐K calc‐alkaline group ( La/Ta ratio >25) that occurs over intermediate thickness lithosphere on the margins of the seismic gap and behind the main CVZ and represents an intermediate percentage of mantle partial melt, and (3) a small‐volume shoshonitic group (very high K) that occurs over relatively thick continental lithosphere in the northeast Puna and Altiplano and represents a very small percentage of mantle partial melt. Mantle‐generated characteristics of these lavas are partially overprinted by mixing with melts of the overlying thickened crust as shown by the presence of quartz and feldspar xenocrysts, negative Eu anomalies (Eu/Eu < 0.90; most < 0.80), and radiogenic Sr (> 0.7055) and Pb and nonradiogenic Nd ( εNd < −0.4) isotopic ratios. Mixing calculations show that the lavas generally contain more than 20% to 25% crustal melt. The eruption of the intraplate group mafic lavas, the change in regional stress orientation, and the high elevation of the southern Puna are suggested to be the result of the late Pliocene mechanical delamination of a block (or blocks) of continental lithosphere (mantle and possibly lowermost crust). The loss of this lithosphere resulted in an influx of asthenosphere that caused heating of the subducting slab and yielded intraplate basic magmas that produced extensive melting at the base of the thickened crust. Heating of the subducting slab led to formation of the seismic gap and trenchward depletion of the slab component. Backarc calc‐alkaline group lavas erupted on the margins of this delaminated block, whereas shoshonitic group lavas erupted over a zone of relatively thick nondelaminated lithosphere to the north.

On the thermo‐mechanical evolution of continental lithosphere

Elastic thickness of continental lithosphere is modified by both mechanical (thickening and thinning) events and thermal evolution. A simple thermo‐mechanical model can account for the processes of tectonic alteration and subsequent thermal reequilibration of lithospheric thermal structure. Mechanical thinning or thickening of the lithosphere directly affects its thickness and flexural rigidity. After a tectonic event, thermal reequilibration proceeds at a rate dependent upon the amount of thinning or thickening and total lithospheric thickness, previous tectonic history, and the heat flux at the base of the lithosphere. Heat flux at the base of the lithosphere is controlled by the efficiency of convective mantle heat delivery. Rapid heat delivery to the base of the lithosphere retards lithospheric thickening, whereas slow conductive heat delivery allows lithospheric thickening at the maximum rate. The relative importance of thermal effects to mechanical alterations of lithospheric elastic thickness depends on the time scale of concern. Very thin lithosphere undergoes rapid thermal evolution such that cooling between tectonic events significantly thickens the lithosphere. For thick lithosphere, conductive heat transport is so slow that only minor changes in thermal structure and no significant changes in elastic thickness take place between tectonic events. Model results with infinite stretching agree with observations of oceanic lithospheric thickness evolution, showing a divergence from a cooling half‐space at about 70 m.y. Model results also agree with independent estimates of continental thermal evolution in the absence of tectonic events. On the longest time scales approaching the age of the Earth, thermal reequilibration is attained, and any tectonic effects are overprinted. The time scale at which the transition between mechanically and thermally dominated elastic thickness evolution occurs depends on the heat transport efficiency of the convecting mantle. For high Nusselt numbers, thermal effects take longer to become important Even for lower Nusselt numbers, mechanical effects still dominate on time scales of 108 years. Crustal radiogenic heat production reduces the equilibrium lithospheric thermal and elastic thickness. It also introduces a dependence of equilibrium thickness on tectonic crustal thinning or thickening. Model results indicate that on time scales of thrust sheet and sedimentary loading in foreland basins, mechanical effects dominate the evolution of elastic thickness except for very thin lithosphere. The model can be applied to the lithosphere beneath foreland basins so that observed basin geometry can be used to constrain tectonic history prior to loading by thrust sheets and basin sediments.

Upper mantle models and the thickness of the continental lithosphere

Analysis of the Early Rise traveltimes in the United States shows that on several of the profiles there are features which can be interpreted as indicating a low-velocity zone above 200 km, i.e. that the continental lithosphere thickness is less than 200 km. The explosion data from the Nevada Test Site, GNOME and Early Rise are all consistent with Lehmann's suggestion that there is a discontinuity at a depth of about 200 km. Comparison of the observed data between 15° and 23° with model calculations shows that the major differences between the western and central-eastern United States upper mantles occur above 200 km. It appears that the only reasonable explanation for the 200 km discontinuity is that it represents the termination of a zone of partial melting.

The influence of second-scale convection on the thickness of continental lithosphere and crust

The effect of sublithospheric convection on the thickness and structure of the continental lithosphere is studied in numerical models assuming different rheologies (Newtonian, non-Newtonian, and temperature and pressure dependent), heat fluxes, and heating modes (bottom versus internal heating). The stagnant regions near the top of the models are identified with the lithosphere. We distinguish a seismic and thermal lithosphere (controlled by temperature), an elastic lithosphere (controlled by viscosity), and a mechanical lithosphere (controlled by strain rate). Strong lateral thickness undulations with a thick lithosphere above the downwelling regions and a thin lithosphere above the upwelling regions develop. The thickness of the elastic lithosphere is about two-thirds of the mechanical lithosphere, while the ratio of the elastic to the thermal lithosphere varies between 0.4 and 0.8. The time dependence of some models (with internal heating and a surface heat flow of 20 mW/m 2) is characterized by long periods (of the order of 1 Ga) close to steady state and short periods (100 Ma) of changing cell patterns and lithospheric thicknesses. Models showing thick lithospheric roots suggest that the mantle beneath some old shields may be associated with cold, slowly downwelling convective flow rather than being firmly attached to the lithosphere. The missing gravity and topography signals from such regions can be explained by elastic bending of the lithosphere and by dynamic adjustment of the Moho. Observed lithospheric thickness variations in Europe and their minimum to maximum spacing agree well with the models, suggesting sublithospheric convection to be the cause. Compared to other observed features, thickness undulations of the lithosphere seem to provide the strongest indications for sublithospherical convection. Relatively low viscosities in the lower crust may lead to a long-term decoupling of surface topography from Moho topography and mantle dynamics. The Moho depth is capable of adjusting to mantle stresses within relaxation times of the order of 100 Ma, while surface topography may flatten out. Such a mechanism may be important in regions with a deep Moho such as Fennoscandia or with a shallow Moho such as the Pannonian Basin, neither region showing a pronounced surface topography.

Geochemistry of the Taitao ophiolite and near-trench intrusions from the Chile margin triple junction

The Taitao ophiolite of southern Chile lies 10 km from the buried extension of the Peru-Chile trench, and less than 50 km from the present position of the Nazca/South America/Antarctica triple junction. Plio-Pleistocene radiometric and paleontologic ages indicate its formation during ridge subduction, and an ultramafic rock, gabbro, sheeted dike, volcanic and sedimentary rock psuedostratigraphy suggests formation by typical accretionary processes for oceanic lithosphere. Yet major and trace element data show that mafic dikes and volcanic units are transitional from MORB to IAT, and there are abundant silicic volcanic units of calc-alkaline character that have high LIL element and light REE concentrations relative to oceanic plagiogranites. Sr and Nd isotopic data are consistent with that of modern oceanic suites, even though having a greater internal variability. Silicic volcanic units show the more enriched Sr and depleted Nd isotopic ratios relative to dike and gabbro samples. In addition to chemical distinctions, paleobathymetric data support a shallow water origin for some of the upper volcanic units and, assuming local compensation, suggest crustal thicknesses of continental proportion. In the vicinity of the Taitao ophiolite, and extending some 40 km landward of the plate margin, are a series of silicic stocks, sills, and plutons that were intruded into the forearc at the time of ridge collision and ophiolite generation. These calc-alkaline I-type granitoids are light REE enriched and have Sr and Nd isotopic compositions similar to those of the main volcanic chain 200 km landward. Chemically, some of the silicic intrusions are indistinguishable from volcanic units of the ophiolite. In general, major, trace, REE, and isotopic variations of both the ophiolite and the distributed intrusions are atypical of simple fractionation trends for basaltic liquids. Intermediate to silicic units lie along mixing hyperbolae between Taitao gabbro and either forearc sediment or metamorphic basement on a NdSr correlation diagram, and these two crustal components support, respectively, either a 10–25% or 5–10% assimilation. Shutdown of magmatism, and therefore probably partial melting as well, appears to occur within 40 km of the trench, roughly spanning the depth interval for the disappearance of the plagioclase-lherzolite stability field as the zone of mantle upwelling is overridden by an increasing thickness of continental lithosphere. A deeper and more landward absence of partial melt related to the subducted ridge is supported by the correlation of the shutoff and re-initiation of arc volcanism over the northern and southern trailing edges of the postulated subcontinental asthenospheric window. Here, as well as elsewhere in the circum-Pacific, the general restriction of magmatism related to ridge subduction to near-trench settings supports a shallow (0 to 15 km) shutoff mechanism for adiabatic decompressive melting and a rather abrupt return to single-phase (solid) convective rise of mantle into an evolving asthenospheric window.

The thickness of the continental lithosphere

Two independent sets of geophysical data are shown to be particularly sensitive to the thickness of the continental lithosphere. Both are connected with the response of the planet to the last deglaciation event of the current ice age. The first set of data consists of relative sea level histories in the age range 0–10 kyr B.P. from sites along the eastern seaboard of North America. Such data appear to require a continental lithospheric thickness in excess of 200 km. The second set of data consists of the ILS record of the motion of the rotation pole over the time range 0–75 years B.P., which reveals a secular drift at the rate of 0.95 (±0.15) deg/106 years toward Hudson Bay. When the viscosity profile of the mantle is fixed to that required to explain free air gravity and relative sea level data from within the ice margin and the observed nontidal acceleration of planetary rotation, then these observations also suggest a continental lithospheric thickness of the same order.

Pleistocene deglaciation and the Earth's rotation: a new analysis

Summary. This paper is concerned with an analysis of the effect of the Pleistocene glacial cycle upon the Earth’s rotation. We demonstrate that two important geophysical observables may be explained as aspects of the rotational response to surface mass loading by ice sheets. These are the astronomically observed non-tidal component of the acceleration of planetary rotation and the secular drift of the rotation pole relative to the surface geography which is evident in the ILS pole path. The former observation is shown to provide an unambiguous constraint upon the viscosity of the planetary mantle and requires a depth dependence of this parameter which is the same as that which has previously been inferred in studies of the relative sea-level variations and free air gravity anomalies associated with post-glacial rebound. The observed secular drift of the pole cannot provide an independent estimate of the viscosity of the mantle because it depends jointly upon mantle viscosity and lithospheric thickness. With the viscosity profile fixed by the rebound data, the observed speed of polar wander suggests a continental lithospheric thickness which is in excess of that appropriate for old ocean basins but nevertheless in accord with independent constraints.