Crustal Thickness Data Research Articles

Cenozoic crustal thickness variations and surface uplift in the southeastern Tibetan Plateau: Insights from detrital zircon trace elements in the Gongga Shan region

The timing and mechanism of uplift in the southeastern Tibet Plateau have long been subjects of debate. The Gongga Shan region, positioned at the convergence of the Songpan-Ganzi, Chuandian, and Yangtze blocks, serves as a crucial vantage point for unraveling the evolutionary history of the southeastern Tibet Plateau. Nevertheless, the uplift history of the Gongga Shan region remains shrouded in uncertainty. By employing multivariate trace element ratios and reconstructing crustal thickness using zircon Eu/Eu*, our research has unveiled distinct periods of crustal thickening and thinning events. Within the time span of ∼240–160 Ma, a discernible thickening trend emerged from ∼35 to ∼45 km between ∼240 and ∼ 215 Ma, followed by a relatively stable phase from ∼215 to ∼170 Ma, and subsequently a decline from ∼170 to ∼160 Ma, reducing the thickness from ∼45 to ∼30 km. In the period from ∼55 to ∼5 Ma, an initial increase in thickness occurred from ∼30 to ∼60 km between ∼55 and ∼ 30 Ma, succeeded by an overall thinning from ∼60 to ∼35 km between ∼30 and ∼ 10 Ma. Moreover, based on our reconstructed crustal thickness data, we have traced the paleo-elevation changes in the Gongga Shan region from the Cenozoic to the present, revealing a fluctuation from ∼0.5 to ∼4.5 km. We attribute the Triassic-Jurassic crustal thickening and subsequent thinning to the Triassic orogeny and post-collision extension following the closure of the Paleo-Tethys Ocean. Our findings further suggest that during the Eocene-Oligocene period, the Gongga Shan region experienced significant crustal thickening and surface uplift, indicative of intense compression. This phenomenon likely resulted from the northeast-directed compression of the Indian-Eurasian plate, causing substantial crustal shortening and thickening in specific areas of the southeastern Tibet Plateau, leading to rapid surface uplift. Subsequently, during the Oligocene-Miocene, the detachment and subsidence of the lithospheric mantle triggered rapid crustal thinning in the Gongga Shan region. This phase was accompanied by relatively stable and slight surface uplift, signifying a shift in the plateau uplift mode of the southeastern Tibet Plateau from compression to lateral extrusion.

Lunar Procellarum KREEP Terrane (PKT) Stratigraphy and Structure with Depth: Evidence for Significantly Decreased Th Concentrations and Thermal Evolution Consequences

Dating from the lunar magma ocean solidification period, the Procellarum KREEP Terrane (PKT) occupies 16% of the surface but has a much higher thorium abundance compared to the rest of the Moon and is thus interpreted to carry 40% of the radioactive elements by volume in the form of an anomalously thick KREEP-rich layer. Subsequent research has focused on the processes responsible for PKT concentration and localization (e.g., degree-1 convection, farside impact basin effects, etc.), and the effect of PKT high-radioactivity localization on lunar thermal evolution (e.g., topography relaxation, mantle heating, late-stage mare basalt generation, etc.). Here we use a stratigraphic approach and new crustal thickness data to probe the nature of the PKT with depth. We find that most PKT characteristics can be explained by sequential impact cratering events that excavated and redistributed to the surface/near-surface a much thinner Th-rich KREEP layer at depth, implying that no anomalous conditions of PKT thickness, radioactive abundances, geodynamics, thermal effects or magma generation are likely to be required as in the previous studies.

Crustal structure and tectonic evolution of the Newfoundland Ridge, Fogo Basin, and southern Newfoundland transform margin

The southern Newfoundland transform margin is a crustal-scale transfer zone that juxtaposes continental crust of the Grand Banks with the oceanic domain of the Fogo Basin, offshore SE Newfoundland. This crustal feature is generally accepted as a left-lateral transfer zone between the much-studied Scotian and Newfoundland margins, but relatively little is known about either its crustal-scale architecture or tectonic evolution. Interpretation of newly available regional seismic data from the Newfoundland Ridge and Fogo Basin, offshore Canada provides evidence for approximately 500 km of left-lateral displacement and associated segmented extension between the Late Triassic (210 Ma) and latest Jurassic (145 Ma) as a result of differential extension rates between the conjugate margins of Nova Scotia–Morocco and Newfoundland (Grand Banks)–Iberia. We show that the southern Newfoundland transform margin represents a 100–150 km wide zone of distributed left-lateral displacement across extended continental crust at the SW edge of the Grand Banks, and that crust beneath the Newfoundland Ridge and Fogo Basin is unlikely to be oceanic in origin. Kinematic restorations of regional cross sections and crustal area balancing provide the basis for map-based reconstructions of the area. Our results, which integrate existing plate reconstructions, crustal thickness data, and established age relationships, help to constrain the early structural evolution of the North Atlantic as the African and North American plates separated during Triassic to Jurassic rifting.

Mapping the Thermal Structure of the Amazon Craton to Constrain the Tectonic Domains

AbstractRheology of solids is primarily a function of temperature. Mapping depth to the bottom of the magnetic layer (DBML), which is assumed as a proxy for the depth to Curie temperature, can be used to constrain the thermal structure of the lithosphere. Heat flow data are commonly used to address the thermal structure of the lithosphere. However, these data are scarce for the Amazon Craton. Therefore, as an alternative, DBML was calculated using fractal magnetic source distribution. The Amazon craton is usually described in terms of geochronological provinces models. Nevertheless, geophysical data are poorly discussed in these models. We provide a DBML model that was integrated with the potential field, seismic tomography, and crustal thickness data to address this issue. Our model varies from 10 to 80 km, and almost 90% of the estimates are in the 20‐50 km range. The recurrence of long‐wavelength features on independent datasets supports the assessment that the model is robust and recovered real geophysical signals. Our model suggests that the limits between Carajás and Bacajás domains should be revisited as well as the limits between Tapajós and Iriri Xingú domains. Moreover, we also correlate the model with main mineral deposits. Further, the identified DBML anomaly suggests the mantle is serpentinized in the eastern Carajás domain. This feature is highly correlated with high p‐wave velocity indicating fertile mantle conditions, thus making this region favorable for world‐class mineral deposits.

Western Mexico seismic source model for the seismic hazard assessment of the Jalisco-Colima-Michoacán region

The Mexican subduction zone, the Gulf of California spreading center, as well as the triple junction point around the Jalisco and the Michoacan Blocks, represents the most active seismogenic belts inducing seismic hazard in the Jalisco-Colima-Michoacan region. Herein, considering such seismotectonic setting, we develop a new seismic source model for the surrounding of Jalisco-Colima-Michoacan to be used as an input in the assessment of the seismic hazard of the region. This new model is based on revised Poissonian earthquake (1787–2018) and focal mechanism (1963–2015) catalogs, as well as crustal thickness data and all information about the geometry of the subducting slabs. The proposed model consists of a total of 37 area sources, comprising all the three different possible categories of seismicity: shallow crustal, interface subduction, and inslab earthquakes. A special care was taken during the delimitation of the boundaries for each area source to ensure that they represent a relatively homogeneous seismotectonic region and to include a relatively large number of earthquakes that enable us to compute as reliable as possible seismicity parameters. Although the sources were delimited following the standard criteria of assessing the probabilistic seismic hazard, they are also characterized in terms of their seismicity parameters (annual rate of earthquakes above Mw 4.0, b-value, and maximum expected magnitude), mean seismogenic depth, as well as the predominant stress regime. The proposed model defines and characterizes regionalized potential seismic sources that can contribute to the seismic hazard at the Jalisco-Colima-Michoacan region, providing the necessary information for seismic hazard estimates.

Do sea level variations influence mid-ocean ridge magma supply? A test using crustal thickness and bathymetry data from the East Pacific Rise

Recent studies suggest that eustatic sea level fluctuations induced by glacial cycles in the Pleistocene influence mantle-melting and volcanic eruptions at mid-ocean ridges, with models predicting variations in oceanic crustal thickness and seafloor bathymetry linked to sea level change. Analyses of seafloor bathymetry have found evidence of significant spectral energy at frequencies consistent with Milanković cycles of 1/23, 1/41, and 1/80-1/120 ka−1. However, other studies emphasize the need for crustal thickness observations to test the “sea level hypothesis”.Here we investigate the hypothesis of climate driven periodicity in mid-ocean ridge magmatism through analysis of a unique bathymetry and crustal thickness dataset derived from a 3D multi-channel seismic investigation of the East Pacific Rise from 9°42' to 57′N. Crustal thickness data spans the last ∼235 ka in age and reveals three axis-parallel zones of 200-800 m thicker crust. The amplitude and spacing of these thick crust ridges, which are most prominent on the east flank of the ridge, are consistent with predictions of sea level modulated mantle melting. Similarly spaced ridges are apparent in the longer duration (470 ka) seafloor bathymetry data. Spectral analysis of these datasets shows peaks centered near 1/80 ka−1 and locally near 1/41 ka−1 on the east flank in both bathymetry and crustal thickness. West flank spectral results show intermittent peaks near 1/100 ka−1 and 1/41 ka−1 in crustal thickness and no coherent peak frequencies in bathymetry data. We attribute differences between the east and west flank to the impacts of spatially variable asymmetric spreading and small changes in the locus of accretion. Observed half spreading rates are dominantly faster to the east with small ridge jumps transferring crust from the west flank. Lagged cross-correlations between sea level and crustal thickness indicate a maximum when the latter is lagged by ∼45 ka, which align thick crust zones with the ∼100 ka periods of lower sea level. Crustal thickness is also directly compared with seafloor bathymetry, indicating a component of compensated topography with RMS relief at the seafloor of 10 to 29% of crustal thickness variations. However, complexity inherited from variable asymmetric spreading and seafloor faulting is also apparent, and results provide new insights into how the crustal accretion filter modulates the recording of magma supply variations in the crust and in seafloor relief.While the significance of the statistical analysis of these ridge records is limited by the short duration of the available crustal thickness dataset and effects of asymmetric spreading, the novel observations of crustal thickness varying at timescales of ∼80-100 ka require a mechanism, and the sea level hypothesis provides a plausible explanation.

Deep-seated thrust faults bound the Mare Crisium lunar mascon

Mare Crisium is composed of a set of volcanic deposits situated in an impact basin on the Moon's near side. The topography of the mare is dominated by an annulus of elevated topography, the inner edge of which is delineated by basin-concentric wrinkle ridges. From a combination of remotely sensed image and topographic data and numerical modeling, we show that the thrust faults that underlie these ridges penetrate up to 20 km in depth, considerably below the base of the mare deposits themselves. Thrust faults of this scale have not heretofore been recognized on the Moon. Mare Crisium sits above a region of uplifted mantle, which contributes to a mass excess beneath the basin, and we demonstrate by comparison with free-air gravity anomaly and derived crustal thickness data for Crisium that the thrust faults structurally bound this elevated mantle material. By means of finite-element models of stresses induced by lithospheric loading within the basin, we argue that the deep-seated thrusts may have been localized by the boundary between the superisostatic mantle material and a sub-isostatic collar of thickened crust that resulted from basin formation and modification shortly after impact. Importantly, numerous other mare-filled mascon basins on the Moon share the same topographic and tectonic characteristics as Crisium, suggesting that they, too, are underlain by deep-seated thrust faults that formed in a similar manner.

Uplift of the Longmen Shan area in the eastern Tibetan Plateau: an integrated geophysical and geodynamic analysis

The mechanism for uplift of the eastern Tibetan Plateau is still a matter of debate. There are two main models: extrusion and crustal flow. These models have been tested by surface observations, but questions about the uplift remain. In addition, the devastating 2008 Mw 7.9 Wenchuan earthquake along the Longmen Shan fault zone (LMSFZ) reminds us that the tectonic activity within eastern Tibet is complex and poses a major natural hazard. This activity is accompanied by dramatic uplift along the LMSFZ, but only minor convergence (<4 mm year–1) against the Sichuan basin is observed. In order to investigate the mechanism for uplift of Longmen Shan (LMS) area, we explored the lithospheric structure across the Songpan–Ganzi terrane (SGT), LMS, and western Sichuan basin by undertaking an integrated analysis of a variety of data including new, logistically challenging controlled-source seismic profiling (reflection and refraction) results, receiver function estimates of crustal thickness, gravity and magnetic data, GPS data, and geologic constraints. Our analysis of crustal structure indicates that the crust is not thick enough to support its current elevation and that the crust is essentially composed of three layers of similar thickness. Thus, based on our crustal structure model, 2D numerical modelling was conducted to investigate uplift mechanisms. The modelling results indicate that the middle crust beneath the SGT is the most ductile layer, which is the key factor responsible for the crustal-scale faulting, earthquake behaviour, and periods of uplift. In addition, the modelling results indicate that the strong Sichuan block acts as a backstop for the thrusting along the LMS and crustal thickening to the west.

Magma production rate along the Ninetyeast Ridge and its relationship to Indian plate motion and Kerguelen hot spot activity

AbstractThe Ninetyeast Ridge, a linear trace of the Kerguelen hot spot in the Indian Ocean, was emplaced on a rapidly drifting Indian plate. Magma production rates along the ridge track are computed using gravity‐derived excess crustal thickness data. The production rates change between 2 and 15 m3/s over timescales of 3–16 Myr. Major variations in magma production rates are primarily associated with significant changes in the Indian plate velocity with low‐production phases linked to high plate velocity periods. The lowest magma production rate (2 m3/s) at 62 Ma is associated with the rapid northward drift of Indian plate under the influence of the Reunion mantle plume. The contemporaneous slowing of the African plate coincides with increase in magma production rate along the Walvis Ridge in the Atlantic Ocean. The present study suggests that variations in the Indian plate motion and frequent ridge jumps have a major role in controlling the magma production, particularly on long‐period cycles (~16 Myr). Short‐period variations (~5 Myr) in magma productions may be associated with intrinsic changes in the plume, possibly due to the presence of solitary waves in the plume conduit.

Receiver function structure beneath four seismic stations in the Sumatra region

We estimated the velocity structure beneath four three-component broad-band seismic stations in the Sumatra region by the joint inversion of teleseismic receiver functions and surface wave group velocities. The stations, part of GEOFON network GE, are located in diverse geologic settings including backarc basins, forearc basins, and the forearc ridge. The stations are distributed roughly trench-parallel, providing samples of conditions along the length of Sumatra. 143 receiver functions were computed by employing an iterative, time-domain deconvolution in order to minimize acausal noise. The teleseismic observations at each station were grouped by back azimuth and ray parameter and then stacked to increase the signal to noise ratios. Surface wave group velocity dispersion was measured for paths across the Sunda block from events to the northeast of the stations. P-wave velocity profiles were derived by inverting the stacks and group velocity data using a linearized time-domain inversion scheme with an interpolated CRUST2.0 global model for input. The results differ significantly from CRUST2.0, and reflect the diversity of geologic environments within which the stations are located. Crustal thicknesses beneath the stations range from 16.0km at the forearc to 30.0km at a backarc basin. These results add to the available velocity and crustal thickness data for the Sumatra region.

Spatial Interpolation Applied a Crustal Thickness in Brazil

The use of spatial interpolation methods of data is becoming increasingly common in geophysical analysis, for that reason, currently, several software already contain many of these methods, allowing more detailed studies. In the present work four interpolation methods are evaluated, for the crustal thickness data of Brazil tectonic provinces, with the intention of making Moho’s map of the regions. The methods used were IDW, Natural Neighbor, Spline and Kriging. We compiled 257 data that constituted a geographic database implemented in the template Postgree PostGIS and were processed using the tools of interpolation located in the Spatyal Analyst Tools program ArcGIS?9 ESRI. Traditional methods, IDW, Natural Neighbor and Spline, generate artifacts in their results, the effects of aim, not consistent with the behavior of crust. Such anomalies are generated because of mathematical formulation methods added to data compiled gravimetry. The analysis results of geostatistical Kriging are more refined and consistent, showing no specific anormalities, i.e., the crustal thickness variation (thinning and thickening) is introduced gradually. Initial our estimates were separated in four specific blocks. With the approval of new networks (BRASIS, RSISNE and RSIS), the crustal thickness database for Brazil may be amended or supplemented so that new models may be generated more consistently, complementing studies of regional tectonics evolution and seismicity.

Ductile crustal flow in Europe's lithosphere

Potential gravity theory (PGT) predicts the presence of significant gravity-induced horizontal stresses in the lithosphere associated with lateral variations in plate thickness and composition. New high resolution crustal thickness and density data provided by the EuCRUST-07 model are used to compute the associated lateral pressure gradients (LPG), which can drive horizontal ductile flow in the crust. Incorporation of these data in channel flow models allows us to use potential gravity theory to assess horizontal mass transfer and stress transmission within the European crust. We explore implications of the channel flow concept for a possible range of crustal strength, using end-member ‘hard’ and ‘soft’ crustal rheologies to estimate strain rates at the bottom of the ductile crustal layers. The models show that the effects of channel flow superimposed on the direct effects of plate tectonic forces might result in additional significant horizontal and vertical movements associated with zones of compression or extension. To investigate relationships between crustal and mantle lithospheric movements, we compare these results with the observed directions of mantle lithospheric anisotropy and GPS velocity vectors. We identify areas whose evolution could have been significantly affected by gravity-driven ductile crustal flow. Large values of the LPG are predicted perpendicular to the axes of European mountain belts, such as the Alps, Pyrenees–Cantabrian Mountains, Dinarides–Hellenic arc and Carpathians. In general, the crustal flow is directed away from orogens towards adjacent weaker areas. Gravitational forces directed from areas of high gravitational potential energy to subsiding basin areas can strongly reduce lithospheric extension in the latter, leading to a gradual late stage inversion of the entire system. Predicted pressure and strain rate gradients suggest that gravity driven flow may play an essential role in European intraplate tectonics. In particular, in a number of regions the predicted strain rates are comparable to tectonically induced strain rates. These results are also important for quantifying the thickness of the low viscosity zones in the lowermost part of the crustal layers.

Imaging Yellowstone plume‐lithosphere interactions from inversion of ballistic and diffusive Rayleigh wave dispersion and crustal thickness data

Diffusive and ballistic Rayleigh wave dispersion data from three PASSCAL seismic deployments are combined with crustal thickness constraints from receiver function analysis to produce a high‐resolution shear velocity image of the Yellowstone hot spot track crust and uppermost mantle. This synoptic image shows the following crustal features: the eastern Snake River Plain (ESRP) high‐velocity midcrustal layer, low‐velocity lower crust beneath the 4.0–6.6 Ma Heise caldera field, high‐velocity lower crust beneath the &lt;2.1 Ma Yellowstone Calderas, and low‐velocity upper crustal volume beneath the &lt;2.1 Ma Yellowstone caldera fields. The low‐velocity lower crust beneath the 4.0–6.6 Ma Heise caldera field is found to extend outward 50–80 km from the ESRP margins, consistent with outflow of the magmatically heated and thickened ESRP lower crust. In addition, the lack of 10 km of crustal thickening of the ESRP crust, associated with the estimated 10 km of magmatic thickening, requires that the ESRP lower crust has flowed outward in a complex fashion governed by preexisting lower crustal strength heterogeneity. Within the northern Wyoming province, the so‐called 7.x km/s lower crustal magmatic layer is found to extend westward to the N‐S oriented pre‐Cambrian rift margin. The high‐velocity, hence high‐density, 7.x layer beneath the &lt;2.1 Ma Yellowstone caldera fields has apparently inhibited heating of the subcaldera lower crust and instead magmatic heat and fluids are exchanged with the country rock above 13 km depth. The narrow 80 km diameter plume imaged by body wave tomograms, after being sheared to horizontal by plate drift, is manifest as a very low velocity (3.9 km/s) layer that is only about 110 km wide. The ESRP mantle lithosphere has been thinned to about 28 km thickness by the plume's transport of heat and magma upward, lateral advection of the lower lithosphere by plume shear, and ongoing lithospheric dilatation.

Buried impact basin distribution on Mars: Contributions from crustal thickness data

Crustal thickness data (derived from Mars Global Surveyor (MGS) gravity field and topographic data) exposes a number of circular thin areas (CTAs) that may represent deeply buried impact basins, which are often not visible in topography alone. A data set which combines quasi‐circular depressions (QCDs) revealed by the Mars Orbiter Laser Altimeter (MOLA) on the MGS spacecraft with a population of non‐QCD CTAs is a better estimate of the true crater retention ages of the buried surfaces of Mars. This study finds that all regions have older crater retention ages than previously thought based on QCDs alone. The highlands and lowlands appear to have the same basement crater retention age, but Tharsis is younger.

Crustal thickness in Australia: where, how and what for?

Geoscience Australia regularly undertakes crustal thickness measurements and it is the only National Agency that maintains a database of such measurements in Australia and its offshore territories, including the Australian Antarctic Territory. The most reliable estimates of crustal thickness come from refraction and wide-angle reflection seismic experiments, as well as from receiver function analyses. As of September 2007, there are only 474 crustal thickness measurements stored in Geoscience Australia?s database derived from such experiments. Crustal thickness data are essential for development of crustal scale tectonic models, assessment of geohazards, constraining of potential field inversions, estimation of crustal stretching rate in sedimentary basin modelling. Estimates of relative significance of depth of subsidence, basement heat production, and thickness of the crust for positioning of hydrocarbon maturation windows show that effects of crustal thickness can outweigh those of basement heat production.

A Large Basin on the Near Side of the Moon

The differences between the surface structure of the near side and the far side of the Moon have been topics of interest ever since photographs of the far side have been available. One recurrent hypothesis is that a large impact on the near side has deposited ejecta on the far side, resulting in thicker crust there. Specific proposals were made by P.H. Cadogan for the Gargantuan Basin and by E.A. Whitaker for the Procellarum Basin. Despite considerable effort, no consensus has been reached on the existence of these basins. The problem of searching for such a basin is one of finding its signature in a somewhat chaotic field of basin and crater impacts. The search requires a model of the topographic shape of an impact basin and its ejecta field. Such a model is described, based on elevation data of lunar basins collected by the Lidar instrument of the Clementine mission and crustal thickness data derived from tracking Clementine and other spacecraft. The parameters of the model are scaled according to the principles of dimensional analysis and isostatic compensation in the early Moon. The orbital dynamics of the ejecta and the curvature of the Moon are also taken into account. Using such a scaled model, a search for the best fit for a large basin led to identification of a basin whose cavity covers more than half the Moon, including the area of all of the impact basins visible on the near side. The center of this basin is at 22 degrees east longitude and 8.5 degrees north latitude and its average radius is approximately 3,160 km. It is a megabasin, a basin that contains other basins (the far side South Pole-Aitken Basin also qualifies for that designation). It has been called the Near Side Megabasin. Much of the material ejected from the basin escaped the Moon, but the remainder formed an ejecta blanket that covered all of the far side beyond the basin rim to a depth of from 6 to 30 km. Isostatic compensation reduced the depth relative to the mean surface to a range of 1–5 km, but the crustal thickness data reveals the full extent of the original ejecta. The elevation profile of the ejecta deposited on the far side, together with modification for subsequent impacts by known basins (especially the far side South Pole-Aitken Basin) matches the available topographic data to a high degree. The standard deviation of the residual elevations (after subtracting the model from the measured elevations) is about one-half of the standard deviation of the measured elevations. A section on implications discusses the relations of this giant basin to known variations in the composition, mineralogy, and elevations of different lunar terranes.

Formation of lunar mare domes along crustal fractures: Rheologic conditions, dimensions of feeder dikes, and the role of magma evolution

In this study we examine a set of lunar mare domes located in the Hortensius/Milichius/T. Mayer region and in northern Mare Tranquillitatis with respect to their formation along crustal fractures, their rheologic properties, the dimensions of their feeder dikes, and the importance of magma evolution processes during dome formation. Many of these domes display elongated summit vents oriented radially with respect to major impact basins, and several dome locations are also aligned in these preferential directions. Analysis of Clementine UV/VIS and Lunar Prospector gamma ray spectrometer data reveals that the examined mare domes formed from low-Si basaltic lavas of high FeO and low to moderate TiO 2 content. Based on their morphometric properties (diameter, height, volume) obtained by photoclinometric and shape from shading analysis of telescopic CCD images, we derive rheologic quantities (lava viscosity during eruption, effusion rate, duration of the effusion process, magma rise speed) and the dimensions of the feeder dikes. We establish three rheologic groups characterised by specific combinations of rheologic properties and dike dimensions, where the most relevant discriminative parameter is the lava viscosity η. The first group is characterised by 10 4 Pa s < η < 10 6 Pa s and contains the domes with elongated vents in the Milichius/T. Mayer region and two similar domes in northern Mare Tranquillitatis. The second group with 10 2 Pa s < η < 10 4 Pa s comprises the very low aligned domes in northern Mare Tranquillitatis, and the third group with 10 6 Pa s < η < 10 8 Pa s the relatively steep domes near Hortensius and in the T. Mayer region. The inferred dike dimensions in comparison to lunar crustal thickness data indicate that the source regions of the feeder dikes are situated within the upper crust for six of the domes in northern Mare Tranquillitatis, while they are likely to be located in the lower crust and in the upper mantle for the other examined domes. By comparing the time scale of magma ascent with the time scale on which heat is conducted from the magma into the host rock, we find evidence that the importance of magma evolution processes during ascent such as cooling and crystallisation increases with lava viscosity. We conclude that different degrees of evolution of initially fluid basaltic magma are able to explain the broad range of lava viscosities inferred for the examined mare domes. The spectral data reveal that differences in TiO 2 content may additionally account for the systematic difference in lava viscosity between the two examined lunar regions. We show that the described mechanisms are likely to be valid also for other lunar mare domes situated near Cauchy and Arago, regarded for comparison. On the other hand, we find for the Gruithuisen and Mairan highland domes that despite their inferred high lava viscosities of 10 8 – 10 9 Pa s , no significant magma cooling in the dike occurred during ascent, supporting previous findings that the highland domes were formed during a specific phase of non-mare volcanism by highly silicic viscous lavas.

“Plate‐like” subsidence of the East Pacific Rise–South Pacific superswell system

In previous studies the removal of small‐scale features such as seamounts and oceanic islands from bathymetry has revealed a large and unusually shallow region in the South Pacific Ocean, which, at 3000 km wide and up to 1 km high, has been dubbed a “superswell.” These studies use statistical techniques based on finding the modal depth of the bathymetry. Such an analysis, however, does not completely isolate these features, or their associated oceanic plateaus and localized hot spot swells, from the ridge‐generated regional bathymetry upon which they are superimposed. Accordingly, a technique is required that passes beneath topographic constructs rather than through them, as is the tendency of the mean, median, or mode. We have developed an algorithm, MiMIC, that reproducibly removes all these features and reveals the large‐scale bathymetric trends in a manner based upon and consistent with manual interpretation. Application of the algorithm to bathymetry data in the southwest Pacific shows that the depth anomaly with respect to a cooling plate model changes steadily from being too deep at the East Pacific Rise (EPR) crest to being too shallow at the superswell. The largest shallow anomaly of 712 ± 66 m occurs at 98 Ma, not 1300 m at 65 Ma, as has been previously suggested. Most significantly, the superswell appears to be part of a large‐scale, “plate‐like,” subsidence that extends to the EPR crest, rather than an isolated shallowing that reverses the subsidence and causes uplift. We interpret the plate‐like subsidence as due in part to cooling of the oceanic lithosphere and in part to a lateral temperature gradient in the underlying asthenosphere which is maintained by the flow of relatively hot material from beneath the superswell toward the relative cold material beneath the EPR. The best fit model implies a lateral temperature gradient of 0.014°C km−1 and is in general accord with the available effective elastic thickness, crustal thickness, heat flow, and seismic tomography data.

The importance of water to oceanic mantle melting regimes.

The formation of basaltic crust at mid-ocean ridges and ocean islands provides a window into the compositional and thermal state of the Earth's upper mantle. But the interpretation of geochemical and crustal-thickness data in terms of magma source parameters depends on our understanding of the melting, melt-extraction and differentiation processes that intervene between the magma source and the crust. Much of the quantitative theory developed to model these processes has neglected the role of water in the mantle and in magma, despite the observed presence of water in ocean-floor basalts. Here we extend two quantitative models of ridge melting, mixing and fractionation to show that the addition of water can cause an increase in total melt production and crustal thickness while causing a decrease in mean extent of melting. This may help to resolve several enigmatic observations in the major- and trace-element chemistry of both normal and hotspot-affected ridge basalts.

The transition from linear to diffuse plate boundary in the Azores–Gibraltar region: results from a thin-sheet model

We use the thin-sheet plane-stress approach to study the present-day dynamic behavior of the plate boundary between Eurasia and Africa along the Azores–Gibraltar region. This plate boundary, which extends from the Azores triple junction to the Gibraltar strait, shows a tectonic regime that changes from transtension in the west to transpression in the east, with a strike–slip motion in its central segment. Seismological data reveal that the western and central segments are currently marked by a linear series of earthquakes indicating that the plate boundary is located in a narrow zone. In contrast, the eastern segment is not so well defined and deformation spreads over a much broader area. To apply the thin-sheet approach, we combined heat flow, elevation and crustal thickness data to calculate the steady-state geotherm and the total strength of the lithosphere. Several models with different fault friction coefficients and geometries at the eastern segment of the plate boundary were tested. Results are compared with the maximum compressive stress directions from the World Stress Map, and the calculated seismic strain rates and slip vectors from earthquake data. The best fitting models are consistent with the rotation pole of Argus et al. [D.F. Argus et al., J. Geophys. Res. 94 (1989) 5585–5602], and show that the rheological behavior of the plate boundary must necessarily change from the western and central segments to the eastern segment. The diffuse character of the plate boundary east of the Gorringe Bank is dominated by the transition from oceanic to continental lithosphere, the weakness of the Alboran domain, and the convergence between the African and the Eurasian plates. The displacement of the Alboran domain relative to the African plate may play a major role in stress propagation through the Iberian Peninsula and its Atlantic margin.