Jun Korenaga

Crustal structure of the southeast Greenland margin from joint refraction and reflection seismic tomography

We present results from a combined multichannel seismic reflection (MCS) and wideangle onshore/offshore seismic experiment conducted in 1996 across the southeast Greenland continental margin. A new seismic tomographic method is developed to jointly invert refraction and reflection travel times for a two-dimensional velocity structure. We employ a hybrid ray-tracing scheme based on the graph method and the local ray-bending refinement to efficiently obtain an accurate forward solution, and we employ smoothing and optional damping constraints to regularize an iterative inversion. We invert 2318 Pg and 2078 PmP travel times to construct a compressional velocity model for the 350-km-long transect, and a long-wavelength structure with strong lateral heterogeneity is recovered, including (1) ∼30-km-thick, undeformed continental crust with a velocity of 6.0 to 7.0 km/s near the landward end, (2) 30- to 15-km-thick igneous crust within a 150-km-wide continent-ocean transition zone, and (3) 15- to 9-km-thick oceanic crust toward the seaward end. The thickness of the igneous upper crust characterized by a high-velocity gradient also varies from 6 km within the transition zone to ∼3 km seaward. The bottom half of the lower crust generally has a velocity higher than 7.0 km/s, reaching a maximum of 7.2 to 7.5 km/s at the Moho. A nonlinear Monte Carlo uncertainty analysis is performed to estimate the a posteriori model variance, showing that most velocity and depth nodes are well determined with one standard deviation of 0.05–0.10 km/s and 0.25–1.5 km, respectively. Despite significant variation in crustal thickness, the mean velocity of the igneous crust, which serves as a proxy for the bulk crustal composition, is surprisingly constant (∼7.0 km/s) along the transect. On the basis of a mantle melting model incorporating the effect of active mantle upwelling, this velocity-thickness relationship is used to constrain the mantle melting process during the breakup of Greenland and Europe. Our result is consistent with a nearly constant mantle potential temperature of 1270–1340°C throughout the rifting but with a rapid transition in the style of mantle upwelling, from vigorous active upwelling during the initial rifting phase to passive upwelling in the later phase.

Mantle thermal structure and active upwelling during continental breakup in the North Atlantic

Seismic reflection and refraction data acquired on four transects spanning the Southeast Greenland rifted margin and Greenland^Iceland Ridge (GIR) provide new constraints on mantle thermal structure and melting processes during continental breakup in the North Atlantic. Maximum igneous crustal thickness varies along the margin from s 30 km in the near-hotspot zone (6 500 km from the hotspot track) to V18 km in the distal zone (500^1100 km). Magmatic productivity on summed conjugate margins of the North Atlantic decreases through time from 1800 ˛ 300 to 600 ˛ 50 km 3 /km/Ma in the near-hotspot zone and from 700 ˛ 200 to 300 ˛ 50 km 3 /km/Ma in the distal zone. Comparison of our data with the British/Faeroe margins shows that both symmetric and asymmetric conjugate volcanic rifted margins exist. Joint consideration of crustal thickness and mean crustal seismic velocity suggests that along-margin changes in magmatism are principally controlled by variations in active upwelling rather than mantle temperature. The thermal anomaly (vT) at breakup was modest (V100^125‡C), varied little along the margin, and transient. Data along the GIR indicate that the potential temperature anomaly (125 ˛ 50‡C) and upwelling ratio (V4 times passive) of the Iceland hotspot have remained roughly constant since 56 Ma. Our results are consistent with a plume^impact model, in which (1) a plume of radius V300 km and vT of V125‡C impacted the margin around 61 Ma and delivered warm material to distal portions of the margin; (2) at breakup (56 Ma), the lower half of the plume head continued to feed actively upwelling mantle into the proximal portion of the margin; and (3) by 45 Ma, both the remaining plume head and the distal warm layer were exhausted, with excess magmatism thereafter largely confined to a narrow (6 200 km radius) zone immediately above the Iceland plume stem. Alternatively, the warm upper mantle layer that fed excess magmatism in the distal portion of the margin may have been a pre-existing thermal anomaly unrelated to the plume. fl 2001 Elsevier Science B.V. All rights reserved.

Archean Geodynamics and the Thermal Evolution of Earth

Possible geodynamic regimes that may have prevailed in the Archean are investigated by back-tracking the thermal history of Earth from the present-day conditions. If the temporal evolution of plate-tectonic convection is modulated by strong depleted lithosphere created at mid-ocean ridges, more sluggish plate tectonics is predicted when the mantle was hotter, contrary to commonly believed, more rapid tectonics in the past. This notion of sluggish plate tectonics can simultaneously satisfy geochemical constraints on the abundance of heat-producing elements and petrological constraints on the degree of secular cooling, in the framework of simple whole-mantle convection. The geological record of supercontinents back to ∼2.7 Ga is shown to be broadly consistent with the accelerating plate motion as predicted by the new model. Furthermore, the very fact of repeated continental aggregation indicates that thicker depleted lithosphere in the past needs to move more slowly to become negatively buoyant by thermal contraction and also needs to be strong enough to support resulting thermal boundary layer. The concept of many small plates covering Archean ocean basins is thus physically implausible. As a consequence of reduced heat flux in the past, mantle plumes are expected to have been weaker in the Archean. The chemical evolution of Earths mantle may have been encumbered by sluggish plate tectonics and weak mantle plumes, maintaining its compositional heterogeneity at various spatial scales to the present day. Internal heat production probably played an important role in controlling plate dynamics in the early Archean, for which a different mode of mantle convection is suggested.

Origin of gabbro sills in the Moho transition zone of the Oman ophiolite: Implications for magma transport in the oceanic lower crust

The Moho transition zone (MTZ) of the Oman ophiolite commonly includes a number of gabbro sills surrounded by dunites. The petrology and geochemistry of these sills are investigated to provide constraints on how magma migrates from the subridge mantle to the oceanic crust. The gabbro sills have millimeter-scale to tens of centimeter-scale modal layering that closely resembles layering in lower crustal gabbros of the ophiolite. Variations in mineral compositions correlate with the modal layering, but there are no overall trends within the sills. The gabbroic sills and the layered gabbros have clear covariations among mineral compositions, which can be interpreted as a fractional crystallization path from a common parental magma. Together with constraints from mid-ocean ridge thermal evolution and crustal accretion dynamics, the petrological and geochemical observations on the gabbro sills indicate that they formed from small, open-system, melt-filled lenses within the MTZ. The thermal evolution of the MTZ melt lenses, buffered by the ambient mantle, is characterized by a slow cooling rate (<10−3°C/yr) and a small temperature difference (∼0.1–1°C) within lenses. Internal origins for modal layering, such as gravity currents and oscillatory nucleation, are unlikely in such a thermal environment, and we propose that open-system evolution of the melt lenses is essential to produce the observed layering. The formation of the MTZ melt lenses may be a consequence of porous flow with low Peclet number entering a conductively cooling regime, where porosity becomes “clogged” by crystallized plagioclase. Preservation of fine-scale vertical variation in mineral composition, together with correlated compositions of different minerals, rules out diffuse porous flow as the primary mechanism of melt transport above these melt lenses. Instead, melt extraction must have been focused into porous channels or melt-filled fractures. Melt lenses drained by fractures would experience repetitious expulsion with continuous melt replenishment. Modal layering could develop through the expulsion cycles, probably via in situ crystallization at the margins of melt lenses.

Isostatic response of the Australian lithosphere: Estimation of effective elastic thickness and anisotropy using multitaper spectral analysis

Gravity and topography provide important insights regarding the degree and mechanisms of isostatic compensation. The azimuthally isotropic coherence function be- tween the Bouguer gravity anomaly and topography evolves from high to low for increasing wavenumber, a diagnostic that can be predicted for a variety of lithospheric loading models and used in inversions for flexural rigidity thereof. In this study we investigate the isostatic response of continental Australia. We consider the effects of directionally anisotropic plate strength on the coherelce. The anisotropic coherence function is calculated for regions of Australia that have distinctive geological and geophysical properties. The coherence estimation is performed by the Thomson multiple-Slepian-taper spectral analysis method extended to two-dimensional fields. Our analysis reveals the existence of flexural anisotropy in central Australia, indicative of a weaker N-S direction of lower Te. This observation is consistent with the suggestion that the parallel faults in that area act to make the lithosphere weaker in the direction perpendicular to them. It can. also be related to the N-S direction of maximum stress and possibly the presence of E-W running zones weakened due to differential sediment burial rates. We also demonstrate that the multitaper method has distinct advantages for computing the isotropic coherence function. The ability to make many independent estimates of the isostatic response that are minimally affected by spectral leakage results in a coherence that is more robust than with modified periodogram methods, particularly at low wavenumbers. Our analysis elucidates the reasons for discrepancies in previous estimates of effective elastic thickness Te of the Australian lithosphere. In isotropic inversions for Te, we obtain values that are as much as a factor of 2 less than those obtained in standard inversions of the periodogram coherence using Bouguer gravity and topography but greater than those obtained by inversions that utilize free-air rather than Bouguer gravity and ignore the presence of subsurface loads. However, owing to the low spectral power of the Australian topography, the uncertainty on any estimate of Te is substantial.

Methods for resolving the origin of large igneous provinces from crustal seismology

Received 22 August 2001; revised 5 February 2002; accepted 10 February 2002; published 10 September 2002. [1] We present a new quantitative framework to understand the process of mantle melting based on the velocity structure of igneous crust. Our approach focuses on the lower crustal section, which is expected to be least affected by porosity and seawater alteration, especially for thick igneous crust. Our methodology is thus best for constraining the origin of large igneous provinces. First, a quantitative relation between bulk crustal velocity and mantle melting parameters is established on the basis of data from mantle melting experiments. Second, we show how lower crustal velocity can be used to place bounds on the expected range of bulk crustal velocity, despite ambiguity in crustal emplacement processes. By modeling fractional crystallization processes at a range of crustal pressures, these bounds are derived as a function of the proportion of lower versus upper crust. Finally, a simple mantle melting model is constructed to illustrate the effects of potential temperature, active upwelling, and a preexisting lithospheric lid on predicted crustal thickness and velocity. As an example, this new interpretation is applied to a seismic transect across the southeast Greenland margin to constrain mantle dynamics during the opening of the North Atlantic. Some complicating factors in seismological inference on mantle melting process, such as the possibilities of subcrustal igneous fractionation and mantle source heterogeneity, are also discussed with this example. INDEX TERMS: 7220 Seismology: Oceanic crust; 3640 Mineralogy and Petrology: Igneous petrology; 3035 Marine Geology and Geophysics: Midocean ridge processes; 8121 Tectonophysics: Dynamics, convection currents and mantle plumes; KEYWORDS: lower crust, mantle melting, mantle plumes, igneous fractionation, mantle source heterogeneity, North Atlantic igneous province

Major element heterogeneity in the mantle source of the North Atlantic igneous province

High-MgO (s 8.5 wt%), aphyric lavas erupted at various locations in the North Atlantic igneous province are utilized to characterize the nature of mantle melting during the formation of this province. Based on the observation that the Ni concentration in residual mantle olivine mostly falls in the range of 2000^3500 ppm, these high-MgO samples are corrected for olivine fractionation until the Ni concentration of equilibrium olivine reaches 3500 ppm, to estimate the composition of primary mantle-derived melt. Estimated primary melt compositions suggest that this province is characterized by significant major element source heterogeneity possibly resulting from basalt addition prior to melting. Primary melts for Southwest Iceland and Theistareykir (North Iceland) are shown to require different source mantle compositions. Whereas the Theistareykir primary melt may be explained by the melting of pyrolitic mantle, the source mantle for Southwest Iceland must be enriched in iron, having molar Mg/(Mg+Fe), or Mg#, 6 0.88. This compositional dichotomy in Iceland seems to continue to adjacent Mid-Atlantic Ridge segments, i.e. the Kolbeinsey and Reykjanes Ridges. The primary melts for East and Southeast Greenland also indicate a fertile mantle source, and the estimate of Mg# is the lowest for the East Greenland source mantle (6 0.87). The inferred spatial extent of source heterogeneity suggests the presence of a long-lived compositional anomaly in this igneous province since the opening of the North Atlantic. fl 2000 Elsevier Science B.V. All rights reserved.

Melt migration through the oceanic lower crust: a constraint from melt percolation modeling with finite solid diffusion

Abstract We present a melt percolation model incorporating finite solid diffusion to provide a quantitative constraint on how melt migrates through the oceanic lower crust at fast-spreading ridge axes. The lower crustal, layered gabbro in the Oman ophiolite, which was formed at a fast-spreading ridge, shows correlated variations in primary mineral compositions with a vertical wavelength less than 100–200 m. Possible effects of porous melt flow on these compositional variations are considered. The results of our numerical modeling indicate that melt transport by porous flow must be less than a few percent of the total incoming melt flux from the sub-ridge mantle, in order to preserve the observed correlations between different mineral compositions. It is also shown that rapid damping of compositional variations due to finite solid diffusion is likely, further reducing the potential role of porous flow melt transport. The dominant mode of melt migration through the oceanic lower crust must be more focused than pervasive porous flow, such as ascent in melt-filled fractures.

On the Likelihood of Plate Tectonics on Super-Earths: Does Size Matter?

The operation of plate tectonics on Earth is essential to modulate its atmospheric composition over geological time and is thus commonly believed to be vital for planetary habitability at large. It has been suggested that plate tectonics is very likely for super-Earths, with or without surface water, because a planet with a larger mass tends to have sufficient convective stress to escape from the mode of stagnant-lid convection. Here, this suggestion is revisited on the basis of the recently developed scaling laws of plate-tectonic convection, which indicate that the planetary size plays a rather minor role and that the likelihood of plate tectonics is controlled largely by the presence of surface water.

Mantle mixing and continental breakup magmatism

Abstract The frequent formation of large igneous provinces during the opening of the Atlantic Ocean is a surface manifestation of the thermal and chemical state of convecting mantle beneath the supercontinent Pangea. Recent geochemical and geophysical findings from the North Atlantic igneous province all point to the significant role of incomplete mantle mixing in igneous petrogenesis. On the basis of a whole-mantle convection model with chemical tracers, I demonstrate that sublithospheric convection driven by surface cooling can bring up dense fertile mantle without a thermal anomaly. When small-scale convection in the upper mantle breaks down into the lower mantle, strong counter upwelling takes place, entraining a large amount of dense crustal fragments accumulated at the base of the mantle transition zone. This multi-scale mantle mixing could potentially explain a variety of hotspot phenomenology as well as the formation of both volcanic and non-volcanic rifted margins, with a spatially and temporally varying distribution of fertile mantle.