Showing papers on "Mid-ocean ridge published in 2009"

Cretaceous ridge subduction along the lower yangtze river belt, eastern china

Abstract: The Lower Yangtze river belt is one of the most important metallogenic belts in China. The mechanisms responsible for ore genesis and the formation of related Cretaceous igneous rocks, such as adakite, A-type granitoid, and Nb-enriched basalt, remain controversial. Mesozoic granitoids in the Lower Yangtze river belt were mostly formed in the Early Cretaceous (140–125 Ma), and three granitoid belts—the inner, the south, and the north—have been defined according to petrological and geochemical characteristics. Previously, based mainly on negative eNd and high initial Sr isotope values, the adakitic rocks were generally attributed to partial melting of thickened or delaminated lower crust, both of which require crustal thickening. Mesozoic crustal thickening, however, is not supported by the development of extensional basins in the region. From the Late Jurassic to Cretaceous, eastern China was closely associated with subduction of the Pacific plate in the south and the Izanagi plate in the north. The midocean ridge (MOR) between these two plates was drifting toward and likely subducting under the Lower Yangtze river belt. A ridge subduction model can therefore explain the distribution of different magmatic rocks and ore deposits in the belt. Partial melting of subducting young, hot oceanic slabs close to the ridge formed adakitic rocks. The negative eNd values of adakitic rocks can be plausibly interpreted by mixing between adakitic magmas and enriched components in the lithospheric mantle, and/or crustal materials through AFC process. A slab window opened during ridge subduction as indicated by A-type granitoids in the center of the inner belt. Nb-enriched basalt found in the belt was likely formed by partial melting of a mantle wedge metasomatized by fluids released from the subducting slab at shallow depths.

Destruction geodynamics of the North China craton and its Paleoproterozoic plate tectonics

Abstract: Much attention has been paid in the last two decades to the physical and chemical processes as well as temporal-spatial variations of the lithospheric mantle beneath the North China craton. In order to provide insights into the geodynamics of this variation, it is necessary to thoroughly study the state and structure of the lithospheric crust and mantle of the North China craton and its adjacent regions as an integrated unit. Based on the velocity structure of the crust and upper mantle constrained from seismological studies, this paper presents various available geophysical results regarding the lithosphere thickness, the nature of crust-mantle boundary, the upper mantle structure and deformation characteristics as well as their tectonic features and evolution systematics. Combined with the obtained data from petrology and geochemistry, a mantle flow model is proposed for the tectonic evolution of the North China craton during the Mesozoic-Cenozoic. We suggest that subduction of the Pacific plate made the mantle underneath the eastern Asian continent unstable and able to flow faster. Such a regional mantle flow system would cause an elevation of melt/fluid content in the upper mantle of the North China craton and the lithospheric softening, which, subsequently resulted in destruction of the North China craton in different ways of delamination and thermal erosion in Yanshan, Taihang Mountains and the Tan-Lu Fault zone. Multiple lines of evidence recorded in the crust of the North China craton, such as the amalgamation of the Archean eastern and western blocks, the subduction of Paleo-oceanic crust and Paleo-continental residue, indicate that the Earth in the Paleoproterozoic had already evolved into the plate tectonic system similar to the present plate tectonics.

An assessment of upper mantle heterogeneity based on abyssal peridotite isotopic compositions

Abstract: [1] Abyssal peridotites, the depleted solid residues of ocean ridge melting, are the most direct samples available to assess upper oceanic mantle composition. We present detailed isotope and trace element analyses of pyroxene mineral separates from Southwest Indian Ridge abyssal peridotites and pyroxenites in order to constrain the size and length scale of mantle heterogeneity. Our results demonstrate that the mantle can be highly heterogeneous to <1 km and even <0.1 m length scales. Examination of Nd isotopes in relation to modal, trace, and major element compositions indicate that the length scales and amplitudes of heterogeneities in abyssal peridotites reflect both ancient mantle heterogeneity and recent modification by melting, melt-rock reaction and melt crystallization. The isotopic and trace element compositions of pyroxenite veins in this study indicate that they are not direct remnants of recycled oceanic crust, but instead are formed by recent melt crystallization. Combined with existing data sets, the results show that the average global isotopic composition of peridotites is similar to that of mid-ocean ridge basalts, though peridotites extend to significantly more depleted 143Nd/144Nd and 87Sr/86Sr. Standard isotope evolution models of upper mantle composition do not predict the full isotopic range observed among abyssal peridotites, as they do not account adequately for the complexities of ancient and recent melting processes.

Mid-Cretaceous seafloor spreading pulse: Fact or fiction?

Abstract: Two main hypotheses compete to explain the mid-Cretaceous global sea-level highstand: a massive pulse of oceanic crustal production that occurred during the Cretaceous Normal Superchron (CNS) and the “supercontinent breakup effect,” which resulted in the creation of the mid-Atlantic and Indian ocean ridges at the expense of subducting old ocean floor in the Tethys and the Pacific. We have used global oceanic paleo-age grids, including now subducted ocean floor and two alternative time scales, to test these hypotheses. Our models show that a high average seafloor spreading rate of 92 mm/a in the Early Cretaceous that decreased to 60 mm/a during the Tertiary, with peaks of 86 mm/a and 70 mm/a at 105 Ma and 75 Ma ago, respectively, correspond to the two observed sea-level highstands in the Cretaceous. Calculations using GTS2004 produce lower seafloor spreading rates during the same period and diminish the mid-Cretaceous spreading pulse. Global ridge lengths increased in the earliest Cretaceous but stayed relatively constant through time. However, we find that the average age of the ocean basins through time is only weakly dependent on the choice of time scale. The expansive mid- and Late Cretaceous epicontinental seas, coupled with warm climates and oxygen-poor water masses, were ultimately driven by the younger average age of the Cretaceous seafloor and faster seafloor spreading rather than a vast increase in mid-ocean ridge length due to the breakup of Pangea or solely by higher seafloor spreading rates, as suggested previously.

Southern African topography and erosion history: plumes or plate tectonics?

Abstract: The physiography of southern Africa comprises a narrow coastal plain, separated from an inland plateau by a horseshoe-shaped escarpment. The interior of the inland plateau is a sedimentary basin. The drainage network of southern Africa is characterized by three river divides, broadly parallel to the coastline. These features contrast strongly with the broad dome and radial drainage patterns predicted by models which ascribe the physiography of southern Africa to uplift over a deep mantle plume. The drainage divides are interpreted as axes of epeirogenic uplift. The ages of these axes, which young from the margin to the interior, correlate closely with major reorganizations of spreading regimes in the oceanic ridges surrounding southern Africa, suggesting an origin from stresses related to plate motion. Successive epeirogenic uplifts of southern Africa on the axes, forming the major river divides, initiated cyclic episodes of denudation, which are coeval with erosion surfaces recognized elsewhere across Africa.

Permeability of asthenospheric mantle and melt extraction rates at mid-ocean ridges

Abstract: The timescale for the segregation and transport of basaltic magma as it rises to form new ocean crust at mid-ocean ridges is critically dependent on the permeability of the partially molten mantle. This parameter is hard to measure, as melt migration is an extremely slow process that occurs at extreme conditions of temperature and pressure. Connolly et al. use a high-pressure, high-temperature centrifuge to measure the rate of basalt melt flow in olivine aggregates as a model of the mantle, and obtain permeabilities that are one to two orders of magnitude greater than predicted by current parameterizations. The inclusion of these permeabilities in mantle models yields transport times of only 1,000–2,500 years for extraction of basaltic melt from the base of the melting region. These timescales are sufficiently rapid to explain the observed 'excesses' of short-lived isotopes such as radium-226 in mid-ocean-ridge basalts. The timescale for segregation and transport of basaltic melts, which are ultimately responsible for formation of the Earth's crust, is critically dependent on the permeability of the partly molten asthenospheric mantle, yet this permeability is known mainly from semi-empirical and analogue models. A high-pressure, high-temperature centrifuge is now used to measure the rate of basalt melt flow in olivine aggregates; the resulting permeabilities are one to two orders of magnitude larger than predicted by current parameterizations. Magmatic production on Earth is dominated by asthenospheric melts of basaltic composition that have mostly erupted at mid-ocean ridges. The timescale for segregation and transport of these melts, which are ultimately responsible for formation of the Earth’s crust, is critically dependent on the permeability of the partly molten asthenospheric mantle, yet this permeability is known mainly from semi-empirical and analogue models1,2,3,4,5,6. Here we use a high-pressure, high-temperature centrifuge, at accelerations of 400g–700g, to measure the rate of basalt melt flow in olivine aggregates with porosities of 5–12 per cent. The resulting permeabilities are consistent with a microscopic model in which melt is completely connected, and are one to two orders of magnitude larger than predicted by current parameterizations4,7. Extrapolation of the measurements to conditions characteristic8 of asthenosphere below mid-ocean ridges yields proportionally higher transport speeds. Application of these results in a model9 of porous-media channelling instabilities10 yields melt transport times of ∼1–2.5 kyr across the entire asthenosphere, which is sufficient to preserve the observed 230Th excess of mid-ocean-ridge basalts and the mantle signatures of even shorter-lived isotopes such as 226Ra (refs 5,11–14).

Propagation of a melting anomaly along the ultraslow Southwest Indian Ridge between 46°E and 52°20′E: interaction with the Crozet hotspot?

Abstract: SUMMARY Regional axial depths, mantle Bouguer anomaly values, geochemical proxies for the extent of partial melting and tomographic models along the Southwest Indian Ridge (SWIR) all concur in indicating the presence of thicker crust and hotter mantle between the Indomed and Gallieni transform faults (TFs; 46 ◦ E and 52 ◦ 20 � E) relative to the neighbouring ridge sections. Accreted seafloor between these TFs over the past ∼10 Myr is also locally much shallower (>1000 m) and corresponds to thicker crust (>1.7 km) than previously accreted seafloor along the same ridge region. Two large outward facing topographic gradients mark the outer edges of two anomalously shallow off-axis domains on the African and Antarctic plates. Their vertical relief (>2000 m locally) and their geometry, parallel to the present-day axis along a >210-kmlong ridge section, suggest an extremely sudden and large event dated between ∼8 (magnetic anomaly C4n) and ∼11 Ma (magnetic anomaly C5n). Asymmetric spreading and small ridge jumps occur at the onset of the formation of the anomalously shallow off-axis domains, leading to a re-organization of the ridge segmentation. We interpret these anomalously shallow off-axis domains as the relicts of a volcanic plateau due to a sudden increase of the magma supply. This event of enhanced magmatism started in the central part of the ridge section and then propagated along axis to the east and probably also to the west. However, it did not cross the Gallieni and Indomed TFs suggesting that large offsets can curtail or even block along-axis melt flow. We propose that this melting anomaly may be ascribed to a regionally higher mantle temperature provided by mantle outpouring from the Crozet hotspot towards the SWIR.

Internal solitary waves induced by flow over a ridge: With applications to the northern South China Sea

Abstract: [1] The generation of internal solitary waves by barotropic tides over a ridge is studied in a nonhydrostatic numerical model under idealized oceanographic settings. The experiments examine the effects of ridge width, barotropic tidal strength, and stratification on wave generation. The barotropic tidal flow produces internal wave beams emitting from the ridge top if the slope of the ridge exceeds a critical value equal to the slope of the wave beam. Reflection and refraction of a wave beam in an upper ocean waveguide associated with a strong shallow thermocline produce horizontally propagating internal tides. When the local Froude number over a ridge is not small, lee waves generated on the ridge convert enough energy from the barotropic tides to the internal tides to form tidal bores and solitary waves. Increasing stratification at ridge depths enhances the generation of internal waves, particularly at the diurnal periods. In the Luzon Strait, the slope of the wave beam decreases in spring and summer as stratification at the ridge depths increases, favoring the generation of internal tides. Without the presence of a strong shallow thermocline, internal solitary waves are not observed east of the Luzon Strait. In the northern South China Sea, internal solitary waves are likely observed from April to July when a strong shallow thermocline is present. A deep mixed layer in winter suppresses the production of internal solitary waves.

Magmatic filtering of mantle compositions at mid-ocean-ridge volcanoes

Abstract: The Earth's mantle constitutes over 80% of the planet's volume, and is therefore a key reservoir in global geochemical cycling. The magnitudes and length scales of heterogeneities in the composition of the mantle are an important aspect of this reservoir, but are inaccessible to direct sampling. Mid-ocean-ridge basalt (MORB), the dominant eruptive product of ridges, provides a geographically widespread first-order snapshot of the mantle in terms of the distribution of major and trace elements and its isotopic composition. However, a range of processes occur between melt generation at depth and eruption on the sea floor that modulate the chemical signals of mantle heterogeneity in MORB, making it an imperfect sample. Detailed observations over the past few years have revealed that regional mantle heterogeneity is generally preserved in MORB most accurately where the melt supply is low, and that timescales between melt generation and MORB eruption are relatively short. Nevertheless, because of the variety of volcanic and magmatic processes that act to preserve or destroy signatures of mantle heterogeneity in MORB, a much broader base of observations from different locations will be required to faithfully reconstruct upper mantle heterogeneity.

La–SiO2 and Yb–SiO2 systematics in mid-ocean ridge magmas: implications for the origin of oceanic plagiogranite

Abstract: Analytical expressions for the variation in D La and D Yb with increasing liquid SiO2 for olivine, plagioclase, augite, hornblende, orthopyroxene, magnetite and ilmenite (Brophy in Contrib Mineral Petrol 2008, online first) have been combined with numerical models of hydrous partial melting, of mid-ocean ridge (MOR) cumulate gabbro melting, and fractional crystallization of slightly hydrous mid-ocean ridge basalt (MORB) magma to assess a melting versus fractionation origin for oceanic plagiogranite. For felsic magmas (>63 wt.% SiO2) the modeling predicts the following. MOR cumulate gabbro melting should yield constant or decreasing La and constant Yb abundances with increasing liquid SiO2. The overall abundances should be similar to those in associated mafic magmas. MORB fractional crystallization should yield steadily increasing La and Yb abundances with increasing SiO2 with overall abundances significantly higher than those in associated mafic magmas. Application to natural occurrences of oceanic plagiogranite indicate that both MOR cumulate gabbro melting and MORB fractionation are responsible. Application of the model results to Icelandic rhyolites strongly support a fractional crystallization rather than a crustal melting origin.

Seismic evidence for thermally‐controlled dehydration reaction in subducting oceanic crust

Abstract: [1] We perform travel-time tomography to estimate detailed seismic velocity structures in the crust of the Pacific slab from northeastern (NE) Japan to the Kanto district, Japan, and reveal that the depth extent of the low-velocity (hydrated) oceanic crust varies along the arc. The low-velocity oceanic crust is subducting to depths of 120–150 km beneath Kanto, which is 40–70 km deeper compared to NE Japan. Such deeper preservation of the low-velocity oceanic crust beneath Kanto can be explained by lower-temperature conditions in the Pacific slab as a result of the subduction of the Philippine Sea slab immediately above it. These observations suggest that dehydration reactions accompanied by large velocity changes are controlled principally by temperatures, not by pressures. We also find spatial correspondence between intensive seismicity in the oceanic crust and the disappearance depth of the low-velocity oceanic crust, suggesting that breakdown of hydrous minerals triggers earthquakes in the oceanic crust.

Zircon Dating of Oceanic Crustal Accretion

Abstract: Most of Earth's present-day crust formed at mid-ocean ridges. High-precision uranium-lead dating of zircons in gabbros from the Vema Fracture Zone on the Mid-Atlantic Ridge reveals that the crust there grew in a highly regular pattern characterized by shallow melt delivery. Combined with results from previous dating studies, this finding suggests that two distinct modes of crustal accretion occur along slow-spreading ridges. Individual samples record a zircon date range of 90,000 to 235,000 years, which is interpreted to reflect the time scale of zircon crystallization in oceanic plutonic rocks.

Seismic reflection images of a near-axis melt sill within the lower crust at the Juan de Fuca ridge

Abstract: The style of accretion of the lower oceanic crust at mid-ocean ridges is disputed, with some models proposing that the lower oceanic crust is accreted from melt sills intruded at multiple levels within the lower crust. Until now, however, seismic images of molten sills within the lower crust have been elusive. Canales et al. report deep crustal seismic reflections off the southern Juan de Fuca Ridge, which they interpret as originating from a molten sill currently forming within the lower oceanic crust. The style of accretion of the lower oceanic crust at mid-ocean ridges is disputed, with some models proposing that the lower oceanic crust is accreted from melt sills intruded at multiple levels within the lower crust. However, seismic images of such sills have been elusive; here, deep crustal seismic reflections off the southern Juan de Fuca ridge are interpreted as originating from a molten sill presently forming within the lower oceanic crust. The oceanic crust extends over two-thirds of the Earth’s solid surface, and is generated along mid-ocean ridges from melts derived from the upwelling mantle1. The upper and middle crust are constructed by dyking and sea-floor eruptions originating from magma accumulated in mid-crustal lenses at the spreading axis2,3,4,5,6, but the style of accretion of the lower oceanic crust is actively debated7. Models based on geological and petrological data from ophiolites propose that the lower oceanic crust is accreted from melt sills intruded at multiple levels between the Moho transition zone (MTZ) and the mid-crustal lens8,9,10,11, consistent with geophysical studies that suggest the presence of melt within the lower crust12,13,14,15,16. However, seismic images of molten sills within the lower crust have been elusive. Until now, only seismic reflections from mid-crustal melt lenses2,17,18 and sills within the MTZ have been described19, suggesting that melt is efficiently transported through the lower crust. Here we report deep crustal seismic reflections off the southern Juan de Fuca ridge that we interpret as originating from a molten sill at present accreting the lower oceanic crust. The sill sits 5–6 km beneath the sea floor and 850–900 m above the MTZ, and is located 1.4–3.2 km off the spreading axis. Our results provide evidence for the existence of low-permeability barriers to melt migration within the lower section of modern oceanic crust forming at intermediate-to-fast spreading rates, as inferred from ophiolite studies9,10.

Melt generation, crystallization, and extraction beneath segmented oceanic transform faults

Abstract: [1] We examine mantle melting, fractional crystallization, and melt extraction beneath fast slipping, segmented oceanic transform fault systems. Three-dimensional mantle flow and thermal structures are calculated using a temperature-dependent rheology that incorporates a viscoplastic approximation for brittle deformation in the lithosphere. Thermal solutions are combined with the near-fractional, polybaric melting model of Kinzler and Grove (1992a, 1992b, 1993) to determine extents of melting, the shape of the melting regime, and major element melt composition. We investigate the mantle source region of intratransform spreading centers (ITSCs) using the melt migration approach of Sparks and Parmentier (1991) for two end-member pooling models: (1) a wide pooling region that incorporates all of the melt focused to the ITSC and (2) a narrow pooling region that assumes melt will not migrate across a transform fault or fracture zone. Assuming wide melt pooling, our model predictions can explain both the systematic crustal thickness excesses observed at intermediate and fast slipping transform faults as well as the deeper and lower extents of melting observed in the vicinity of several transform systems. Applying these techniques to the Siqueiros transform on the East Pacific Rise we find that both the viscoplastic rheology and wide melt pooling are required to explain the observed variations in gravity inferred crustal thickness. Finally, we show that mantle potential temperature Tp = 1350°C and fractional crystallization at depths of 9–15.5 km fit the majority of the major element geochemical data from the Siqueiros transform fault system.

Mid-ocean-ridge basalt of Indian type in the northwest Pacific Ocean basin

Abstract: Since 42 million years ago, the northwestern Pacific Izu Bonin arc magmas have incorporated lead from subducted Indian-type oceanic crust. This crust probably formed at a now-extinct spreading centre in the Pacific basin that tapped Indian-type upper mantle, suggesting a greater extent for this mantle domain than accepted at present.

An experimental study of focused magma transport and basalt–peridotite interactions beneath mid-ocean ridges: implications for the generation of primitive MORB compositions

Abstract: We performed experiments in a piston-cylinder apparatus to determine the effects of focused magma transport into highly permeable channels beneath mid-ocean ridges on: (1) the chemical composition of the ascending basalt; and (2) the proportions and compositions of solid phases in the surrounding mantle. In our experiments, magma focusing was supposed to occur instantaneously at a pressure of 1.25 GPa. We first determined the equilibrium melt composition of a fertile mantle (FM) at 1.25 GPa-1,310°C; this composition was then synthesised as a gel and added in various proportions to peridotite FM to simulate focusing factors Ω equal to 3 and 6 (Ω = 3 means that the total mass of liquid in the system increased by a factor of 3 due to focusing). Peridotite FM and the two basalt-enriched compositions were equilibrated at 1 GPa-1,290°C; 0.75 GPa-1,270°C; 0.5 GPa-1,250°C, to monitor the evolution of phase proportions and compositions during adiabatic decompression melting. Our main results may be summarised as follows: (1) magma focusing induces major changes of the coefficients of the decompression melting reaction, in particular, a major increase of the rate of opx consumption, which lead to complete exhaustion of orthopyroxene (and clinopyroxene) and the formation of a dunitic residue. A focusing factor of ≈4—that is, a magma/rock ratios equal to ≈0.26—is sufficient to produce a dunite at 0.5 GPa. (2) Liquids in equilibrium with olivine (±spinel) at low pressure (0.5 GPa) have lower SiO2 concentrations, and higher concentrations in MgO, FeO, and incompatible elements (Na2O, K2O, TiO2) than liquids produced by decompression melting of the fertile mantle, and plot in the primitive MORB field in the olivine–silica–diopside–plagioclase tetrahedron. Our study confirms that there is a genetic relationship between focused magma transport, dunite bodies in the upper mantle, and the generation of primitive MORBs.

Geochronology and Geochemistry of the Ore-Bearing Porphyries in the Baogutu Area (Western Junggar):Petrogenesis and Their Implications for Tectonics and Cu-Au Mineralization

Abstract: The small porphyry plutons or dikes in the Baogutu area,western Junggar,have attracted wide attentions owing to the close association between them and Cu-Au mineralizationThis paper presents new LA-ICP-MS zircon U-Pb age and geochemical data of ore-bearing porphyries in the Baogutu areaThe quartz diorite porphyry bodies Ⅱ and Ⅴ and diorite porphyry body Ⅲ have crystallization ages of 3149±17Ma,3099±19Ma,and 3139±26Ma,respectively,suggesting they were generated in Late CarboniferousThey are characterized by high Na2O/K2O and high Sr values but low Y and Yb contents,and negligible Eu anomalies,similar to adakitesIn addition,some samples have high MgO(393%-478%)and Mg#(47-74)values,similar to high-Mg andesiteTaking into account the data of regional geology and magmatic rocks,we suggest that(1)The Baogutu intrusive rocks were possibly formed in an island-arc setting linking to ocean ridge suduction in Late Carboniferous,the adakitic magmas have likely formed by partial melting of the leading edge of the subducted ridge,and high-Mg diorites possibly originated from the interaction between adakitic melts and mantle peridotite;(2)The Baogutu Cu-Au mineralization might occur above a slab window during ocean ridge suduction,and the interaction between high oxygen fugacity slab melt and mantle peridotite caused the decomposition of metal sulfides and the Cu and Au mineralization

Models of hydrothermal heat output from a convecting, crystallizing, replenished magma chamber beneath an oceanic spreading center

Abstract: [1] The temperature and heat output of hydrothermal systems at oceanic spreading centers place strong constraints on the mechanism of heat transfer in oceanic crust. In this paper, we investigate time-dependent heat transfer from a vigorously convecting, crystallizing, and replenished magmatic sill beneath an ocean ridge axis, linked to an overlying hydrothermal system. We first consider two different crystallization scenarios, “crystals suspended” and “crystals settling,” coupled with crystallinity-dependent and temperature-dependent magma viscosity. The large-scale convection is assumed to rapidly homogenize the magma, resulting in a characteristic temperature Tm. In cases without magma replenishment, the simulation results for crystal-suspended models show that heat output and the hydrothermal temperature decrease rapidly and that crystallinity reaches 60% in less than 10 a. In crystal-settling models, magma convection may last for decades, but decreasing heat output and hydrothermal temperatures still occur on decadal timescales. When magma replenishment is included, the magmatic heat flux approaches steady state on decadal timescales, whereas the magma body grows to double its original size. The rate of magma replenishment needed ranges between 5 × 105 and 5 × 106 m3 a−1, which is somewhat faster than that required for seafloor spreading but less than that of fluxes to some terrestrial volcanoes on similar timescales. The heat output from a convecting, crystallizing, replenished magma body that is needed to drive observed high-temperature hydrothermal systems is consistent, with gabbro glacier models of crustal production at mid-ocean ridges.

Dynamical modelling of lithospheric extension and small-scale convection: implications for magmatism during the formation of volcanic rifted margins

Abstract: SUMMARY Enhanced melt productivity as a consequence of buoyant upwelling and small-scale convection of the mantle during rifting may play an important role in determining the fundamental structure of igneous crust produced during and following continental breakup. This paper investigates the relationship between rift-related decompression melting and the influence of small-scale mantle convection and rift geometry on the subsequent production and distribution of melt-related crust. Extension of the lithosphere is modelled numerically using a 2-D plane-strain finite element method for viscous-plastic creeping flows. The evolving temperature and pressure fields within the model are coupled to an algorithm that predicts the amount and timing of decompression melting of upwelling mantle. Predicted melt fractions are converted to equivalent thicknesses of igneous crust, and the predicted crustal thicknesses for a series of models are compared to the observed crustal structure of rifted margins inferred from seismic data. Models characterized by small-scale mantle convection can to first order reproduce the general architecture of most volcanic rifted margins, that is, a relatively narrow band of thick (12–13 km) igneous crust (inferred to occur along strike of the margin), juxtaposed with thinner oceanic crust farther offshore. The variability in thickness (4–7 km) predicted for the later-stage thinner igneous crust is however difficult to reconcile with global observations of oceanic crustal thickness (7 ± 1 km). Also, the peak 13 km thickness of igneous crust predicted for models with convectively enhanced upwelling fails to match the great thicknesses (≥20 km) of igneous crust observed at many volcanic margins. Composite models that include both small-scale convection and a small increase to mantle potential temperature predict large pulses in initial magmatism and generation of 17 to 21-km-thick crust, followed by unstable production of thinner igneous crust. The results indicate that models with small-scale convection and no temperature anomaly may play a role in explaining the formation of volcanic margins with only moderately thick (11–15 km) igneous crust. Further, convection coupled with small increases to mantle temperature may be important during the initial phase of very thick igneous crust generation at some volcanic margins. Predicted distributions of igneous crust are moderately sensitive to asymmetric rifting of the lithosphere. Prior to breakup, igneous crust accretion is asymmetric; subsequent to breakup, symmetry in the thermal structure of the upwelling sublithospheric mantle is the dominant control on the final distribution of igneous crust.