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Francis Nimmo - One of the best experts on this subject based on the ideXlab platform.
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Admittance estimates of mean Crustal Thickness and density at the Martian hemispheric dichotomy
Journal of Geophysical Research, 2002Co-Authors: Francis NimmoAbstract:[1] Admittance estimates from line-of-sight (LOS) acceleration profiles of the Mars Global Surveyor spacecraft are used to constrain the mean Crustal Thickness and surface density centered on the hemispheric dichotomy, from 110°E to 220°E, 40°S to 20°N. Models with uniform Crustal properties predict lower than expected bulk Crustal densities. Two-layer models with loading only at the surface and the Moho produce satisfactory fits to the data. The best fit surface density, Crustal Thickness tc, and elastic Thickness Te are 2.5 Mg m−3, 27 km, and 61 km, respectively. Higher elastic Thicknesses require lower Crustal Thicknesses, and vice versa. The best fit ratio of Moho to surface loading F is close to 1. Models with no bottom loading (F = 0) provide a poor fit to the data; underestimates in F result in underestimates of both tc and Te. The surface density is lower than that measured from Martian meteorites and by admittance analyses of young volcanoes but is well constrained by the short-wavelength admittance values. For misfits up to 1.5 times the minimum value and a fixed surface density of 2.5 Mg m−3, the ranges of tc, Te, and F are 1–75 km, 37–89 km, and 0.4–2.6, respectively. The apparently compensated nature of the large impact basins has been used to infer a lower bound on southern hemisphere Crustal Thickness of 45 km. If this estimate is correct, the likely mean Crustal Thickness in the area considered is 55 ± 20 km.
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Constraining the Crustal Thickness on Mercury from viscous topographic relaxation
Geophysical Research Letters, 2002Co-Authors: Francis NimmoAbstract:[1] Mercury exhibits long-wavelength topography which has probably survived for ∼4 Ga. Assuming Airy compensation, the survival of the topography indicates that only certain combinations of Crustal Thickness and thermal structure are allowable. A dry diabase rheology allows a thicker crust than a dry plagioclase rheology. The existence of ancient faults places some constraints on the thermal structure. Unless the crust of Mercury is both as strong as dry diabase and is heated mainly from within, the Crustal Thickness must be ≤200 km. The faulting evidence implies that the concentration of radiogenic elements in the crust plus mantle of Mercury is at least 80% of the terrestrial value. Combined with previous studies of the long wavelength gravity and topography, the Crustal Thickness of Mercury is probably 100–200 km. Faults on Mercury are probably several times stronger than terrestrial faults.
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Estimates of Martian Crustal Thickness from viscous relaxation of topography
Journal of Geophysical Research: Planets, 2001Co-Authors: Francis Nimmo, David J. StevensonAbstract:Isostatically compensated Crustal Thickness variations and associated topographic contrasts at the surface of a planet result in lateral pressure gradients, which may cause the lower crust to flow and reduce the relief. Areas of thicker crust are generally associated with more rapid relaxation of topography. On Mars, topographic features such as impact basins and the hemispheric dichotomy have survived for 4 Gyr. We use a finite difference representation of depth-dependent, non-Newtonian lower Crustal flow to investigate how topography decays with time. For a dry diabase rheology, total radiogenic concentrations ≥80% of terrestrial values, and Crustal radiogenic concentrations similar to terrestrial basalts, we find that an upper bound on the mean planetary Crustal Thickness is ∼100 km. In the probably unrealistic case where all the radiogenic elements are in the crust, this maximum Crustal Thickness can be increased to ∼115 km. The main uncertainty in these results is the total radiogenic abundances on Mars. Comparing our results with the observed shape of the Crustal dichotomy provides no evidence that this slope is primarily the result of lower Crustal flow. Both Hellas and the dichotomy are isostatically compensated; if the mechanism is Airy isostasy, then the lower bound on mean Crustal Thickness is ∼30 km. Crustal Thicknesses of 30–100 km on Mars can be produced by mid-ocean ridge spreading at potential temperatures of 1350°–1600°C. However, for such Crustal Thicknesses the lithosphere is likely to be positively buoyant, making subduction difficult.
Harro Schmeling - One of the best experts on this subject based on the ideXlab platform.
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Crustal accretion at high temperature spreading centres: Rheological control of Crustal Thickness
Physics of the Earth and Planetary Interiors, 2010Co-Authors: Harro SchmelingAbstract:Abstract New determinations of lateral Crustal Thickness variations at anomalous oceanic spreading centres such as Iceland have shown that the crust may be thinner at the ridge axis above the plume thickening towards the sides ( Bjarnason and Schmeling, 2009 ). To understand this behaviour Crustal accretion models have been carried out solving the conservation equations of mass, momentum and energy with melting, melt extraction, and feedback of extracted material as newly formed crust for an extending lithosphere system underlain by a hot mantle plume. The dynamics of rifting are thermally and rheologically controlled by the feedback due to accreted new crust. Four accretional modes with characteristic Crustal Thickness variations are identified depending on the width of the volcanic emplacement zone, the accretional heating rate, which can be associated with the Thickness of the surface layer in which magmatic emplacement takes place, and the spreading rate. Mode 1: zero Crustal Thickness at the spreading axis develops for cool accretion and a wide emplacement zone. Mode 2: strongly or moderately Crustal thickening away from the axis develops in case of warm (deep reaching) accretion and wide emplacement zones. Mode 3: nearly constant Crustal Thickness develops in case of warm (deep reaching) accretion but narrow emplacement zones. Dynamic topography of mode 3 shows only a weak or no regional minimum at all near the axis. Modes 2 or 3 may be identified with the situation in Iceland. Mode 4: a stagnating central Crustal block evolves for cool accretion and narrow emplacement. This mode disappears for increasing spreading rates. No accretional mode with maximum Crustal Thickness above the plume at the rift axis has been found. The absence of mode 1 accretion (with zero crust at ridge axis) on earth may be an indication that in general Crustal accretion is not cold (and shallow). The model is also applied to other hotspot-ridge settings (Azores, Galapagos) and suggests modes 2–3 accretion.
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Crustal accretion at anomalous spreading centres: Rheological control of Crustal Thickness
2010Co-Authors: Harro SchmelingAbstract:New determinations of lateral Crustal Thickness variations at anomalous oceanic spreading centres such as Iceland have shown that the crust may be thinner at the ridge axis above the plume thickening towards the sides (Bjarnason and Schmeling, 2009). To understand this behaviour Crustal accretion models have been carried out solving the conservation equations of mass, momentum and energy with melting, melt extraction, and feed back of extracted material as newly formed crust for an extending lithosphere system underlain by a hot mantle plume. The dynamics of rifting are thermally and rheologically controlled by the feed back due to accreted new crust. Three accretional modes with characteristic Crustal Thickness variations are identified depending on the width of the volcanic emplacement zone and the accretional heating rate. This rate can be associated with the Thickness of the surface layer in which magmatic emplacement takes place. Mode 1: Zero Crustal Thickness at the spreading axis develops for cool accretion and a wide emplacement zone. Mode 2: Constant Crustal Thickness or moderately Crustal thickening to the sides develops in case of warm (deeper reaching) accretion. Dynamic topography shows only a weak or no regional minimum at all near the axis. This mode may be identified with the situation in Iceland. Mode 3: A stagnating central Crustal block evolves for cool accretion and narrow emplacement. No accretional mode with maximum Crustal Thickness above the plume at the rift axis has been found. The absence of mode 1 accretion (with zero crust at ridge axis) on earth may be an indication that in general Crustal accretion is not cold (and shallow).
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Crustal accretion at high temperature spreading centres: Rheological control of Crustal Thickness
Physics of the Earth and Planetary Interiors, 2010Co-Authors: Harro SchmelingAbstract:International audienceNew determinations of lateral Crustal Thickness variations at anomalous oceanic spreading centres such as Iceland have shown that the crust may be thinner at the ridge axis above the plume thickening towards the sides (Bjarnason and Schmeling, 2009). To understand this behaviour Crustal accretion models have been carried out solving the conservation equations of mass, momentum and energy with melting, melt extraction, and feedback of extracted material as newly formed crust for an extending lithosphere system underlain by a hot mantle plume. The dynamics of rifting are thermally and rheologically controlled by the feedback due to accreted new crust. Four accretional modes with characteristic Crustal Thickness variations are identified depending on the width of the volcanic emplacement zone, the accretional heating rate, which can be associated with the Thickness of the surface layer in which magmatic emplacement takes place, and the spreading rate. Mode 1: Zero Crustal Thickness at the spreading axis develops for cool accretion and a wide emplacement zone. Mode 2: Strongly or moderately Crustal thickening away from the axis develops in case of warm (deep reaching) accretion and wide emplacement zones. Mode 3: Nearly constant Crustal Thickness develops in case of warm (deep reaching) accretion but narrow emplacement zones. Dynamic topography of mode 3 shows only a weak or no regional minimum at all near the axis. Mode 2 or 3 may be identified with the situation in Iceland. Mode 4: A stagnating central Crustal block evolves for cool accretion and narrow emplacement. This mode disappears for increasing spreading rates. No accretional mode with maximum Crustal Thickness above the plume at the rift axis has been found. The absence of mode 1 accretion (with zero crust at ridge axis) on earth may be an indication that in general Crustal accretion is not cold (and shallow). The model is also applied to other hotspot-ridge settings (Azores, Galapagos) and suggests mode 2 to 3 accretion
Yongshun John Chen - One of the best experts on this subject based on the ideXlab platform.
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Crustal Thickness anomalies in the north atlantic ocean basin from gravity analysis
Geochemistry Geophysics Geosystems, 2011Co-Authors: Tingting Wang, Jian Lin, Brian E Tucholke, Yongshun John ChenAbstract:Gravity-derived Crustal Thickness models were calculated for the North Atlantic Ocean between 76°N and the Chain Fracture Zone and calibrated using seismically determined Crustal Thickness. About 7% of the ocean crust is 7 km thick and is interpreted to have been affected by excess magmatism. Thin crust probably reflects reduced melt production from relatively cold or refractory mantle at scales of up to hundreds of kilometers along the spreading axis. By far the most prominent thick crust anomaly is associated with Iceland and adjacent areas, which accounts for 57% of total Crustal volume in excess of 7 km. Much smaller anomalies include the Azores (8%), Cape Verde Islands (6%), Canary Islands (5%), Madeira (<4%), and New England–Great Meteor Seamount chain (2%), all of which appear to be associated with hot spots. Hot spot–related Crustal thickening is largely intermittent, suggesting that melt production is episodic on time scales of tens of millions of years. Thickened crust shows both symmetrical and asymmetrical patterns about the Mid-Atlantic Ridge (MAR) axis, reflecting whether melt anomalies were or were not centered on the MAR axis, respectively. Thickened crust at the Bermuda and Cape Verde rises appears to have been formed by isolated melt anomalies over periods of only ∼20–25 Myr. Crustal Thickness anomalies on the African plate generally are larger than those on the North American plate; this most likely results from slower absolute plate speed of the African plate over relatively fixed hot spots.
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Oceanic Crustal Thickness versus spreading rate
Geophysical Research Letters, 1992Co-Authors: Yongshun John ChenAbstract:A compilation of oceanic Crustal Thickness from seismic observations collected over the past two decades shows that the average Crustal Thickness, away from plateaus, is 6 km; no systematic increase of Crustal Thickness with spreading rate is observed. Instead, the data show large variations in Crustal Thickness at slow spreading ridges (3 – 8 km for half rates 30 mm/yr). The large variations at slow ridges and small variations at fast ridges are consistent with the results inferred from recent gravity studies of mid-ocean ridges. Both data sets support the speculation of a transition from a 3-D structure of Crustal accretion at slow ridges to a 2-D accretion pattern at fast ridges.
Robert S. White - One of the best experts on this subject based on the ideXlab platform.
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Variation with spreading rate of oceanic Crustal Thickness and geochemistry
Earth and Planetary Science Letters, 1994Co-Authors: Jonathan W. Bown, Robert S. WhiteAbstract:Away from the influence of fracture zones, hotspots and marginal basins, oceanic crust exhibits remarkably uniform Crustal Thickness, rare earth element concentrations and bulk composition at all but the most slowly spreading ridges. At full spreading rates below 15 mm/a, however, all these observed parameters show marked and abrupt changes. The seismically determined Crustal Thicknesses decrease sharply from their average of 7 ± 1 km, as do melt Thicknesses inferred from rare earth element inversions of basalts. Bulk element compositions of basalts show an increase in the percentage of Na2O, and decreases in the FeO content and CaO/Al2O3 ratio as the full spreading rate drops below 15 mm/a. We present a model of the melting beneath oceanic spreading centres which reproduces the observed variations with spreading rate of Crustal Thickness and the main geochemical changes observed in basalts. Changes in the volume and composition of melt generated at very slow spreading rates are caused by conductive heat loss from the mantle welling up beneath rifts. We model the observed behaviour with normal mantle potential temperatures of 1300 ± 20°C. Greater Crustal Thicknesses are generated at the North Atlantic spreading centre adjacent to the Iceland mantle plume, suggesting mantle temperatures about 80°C hotter than normal in this area.
Jian Lin - One of the best experts on this subject based on the ideXlab platform.
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Crustal Thickness anomalies in the north atlantic ocean basin from gravity analysis
Geochemistry Geophysics Geosystems, 2011Co-Authors: Tingting Wang, Jian Lin, Brian E Tucholke, Yongshun John ChenAbstract:Gravity-derived Crustal Thickness models were calculated for the North Atlantic Ocean between 76°N and the Chain Fracture Zone and calibrated using seismically determined Crustal Thickness. About 7% of the ocean crust is 7 km thick and is interpreted to have been affected by excess magmatism. Thin crust probably reflects reduced melt production from relatively cold or refractory mantle at scales of up to hundreds of kilometers along the spreading axis. By far the most prominent thick crust anomaly is associated with Iceland and adjacent areas, which accounts for 57% of total Crustal volume in excess of 7 km. Much smaller anomalies include the Azores (8%), Cape Verde Islands (6%), Canary Islands (5%), Madeira (<4%), and New England–Great Meteor Seamount chain (2%), all of which appear to be associated with hot spots. Hot spot–related Crustal thickening is largely intermittent, suggesting that melt production is episodic on time scales of tens of millions of years. Thickened crust shows both symmetrical and asymmetrical patterns about the Mid-Atlantic Ridge (MAR) axis, reflecting whether melt anomalies were or were not centered on the MAR axis, respectively. Thickened crust at the Bermuda and Cape Verde rises appears to have been formed by isolated melt anomalies over periods of only ∼20–25 Myr. Crustal Thickness anomalies on the African plate generally are larger than those on the North American plate; this most likely results from slower absolute plate speed of the African plate over relatively fixed hot spots.
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Crustal Thickness of V-shaped ridges south of the Azores: Interaction of the Mid-Atlantic Ridge (36ø-39øN) and the Azores hot spot
Journal of Geophysical Research, 2001Co-Authors: Javier Escartin, Mathilde Cannat, Gaud Pouliquen, Aline Rabain, Jian LinAbstract:V-shaped ridges propagating along the Mid-Atlantic Ridge axis south of the Azores and Iceland hot spots indicate that ridge-hot spot interactions produce temporal and spatial variations in melt supply to the ridge axis. Estimates of relative Crustal Thickness variations associated with the ridges south of the Azores hot spots, based on gravity and bathymetry data collected during the SudAqores cruise (1998), provide constraints on the rate of propagation of these melt anomalies and on the variations in melt production along the axis and in time. The maximum apparent Crustal Thickness along the Azores V ridge is-14 km near the Azores, decreasing to normal Crustal Thickness of-6 km toward the south. This Crustal Thickness variation may be explained by enhanced melt production associated with the propagation of a mantle temperature anomaly that initiated-10 Myr ago at the Azores hot spot. The temperature anomaly decreased as it propagated southward, reaching ambient mantle temperatures at the present time at its predicted location under the axis. The excess melt was emplaced on axis forming discrete, shallow (
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Crustal Thickness and structure along three contrasting spreading segments of the mid atlantic ridge 33 5 35 n
Journal of Geophysical Research, 2000Co-Authors: Emilie E E Hooft, Robert S Detrick, Douglas R Toomey, John A Collins, Jian LinAbstract:The Crustal Thickness and Crustal and upper mantle structure along the rift valleys of three segments of the northern Mid-Atlantic Ridge with contrasting morphologies and gravity signatures are determined from a seismic refraction study. These segments lie between the Oceanographer and Hayes transforms and from north to south have progressively deeper axial valleys with less along-axis relief and smaller mantle Bouguer gravity lows. Major variations in seismic Crustal Thickness and Crustal velocity and density structure are observed along these segments. The thickest crust is found near the segment centers, with maximum Crustal Thicknesses of 8.1, 6.9, and 6.6±0.5 km, decreasing from north to south. However, the mean Crustal Thickness is similar for each segment (5.6±0.4, 5.7±0.4 and 5.1±0.3 km). Near the segment ends, Crustal Thickness is 2.5 to 5±0.5 km with no systematic variation from north to south. At segment ends, both Crustal velocities and vertical velocity gradients are anomalous and may indicate fracturing and alteration of thin igneous crust and underlying mantle. Away from segment ends, the Thickness of the upper crust is relatively uniform along axis (∼3 km), although its internal structure is laterally heterogeneous (velocity anomalies of ±0.6 km s−1 over distances of 5 km), possibly related to the presence of discrete volcanic centers. The along-axis Crustal Thickness variations are primarily accommodated in the lower crust. The center of the northern segment (OH-1) has an unusually thick Crustal root (excess Thickness of 2–4 km and along-axis extent of 12 km). Our results are consistent with an enhanced supply of melt from the mantle to the segment centers and redistribution of magma along axis at shallow Crustal levels by lateral dike injection. Along this portion of the Mid-Atlantic Ridge, our results suggest that differences in axial morphology, seismic Crustal Thickness, and gravity anomalies are correlated and the result of variations in melt flux from the mantle. A surprising result is that the melt flux per segment length is similar for all three segments despite their different morphologies and gravity signatures. This argues against excess melting of the mantle beneath segment OH-1. Instead, we suggest that the thickened crust at the segment center is a result of focusing of melt, possibly due to the influence of the thermal structure of the Oceanographer fracture zone on melt migration in the mantle.