Moho Level Research Articles

Sensing the global CRUST1.0 Moho by gravitational curvatures of crustal mass anomalies

ABSTRACT The gravitational curvatures (GC, the third-order derivatives of the gravitational potential) in gravity field modeling are gaining increased interest in geosciences. The crustal effects of the GC and Moho variation sensed by the GC are not fully evaluated for the current study. In this contribution, the effects of the GC induced by topographic and anomalous crustal masses are investigated based on ETOPO1 and CRUST1.0 models using the tesseroids. By adopting the gravitational stripping correction, the residual GC sensed by the CRUST1.0 Moho depths are presented globally to examine whether the GC can sense crustal mass anomalies at the satellite altitude of 260 km . The spatial analysis using the Pearson correlations coefficient (PCC) between the residual GC and the CRUST1.0 Moho depths is performed. Among the 10 residual GC functionals, the PCC value of the residual radial-radial-radial component δ T zzz res is largest with 0.31, where this value is highly dependent on the spectral content removed from the EGM2008, e.g. signals assumed to relate to deeper mass anomalies. Numerical experiments show that with the increased order of the derivatives up to third-order, the fineness level of different global Moho sensed crustal mass anomalies increases correspondingly. Taking the Tibetan plateau for example, the values of the δ T zzz res can better reveal the detailed features of the Tibetan plateau’s Moho depth than these of the lower-order residual radial functionals (i.e. disturbing potential δ T res , disturbing radial gravity vector δ T z res , and disturbing radial-radial gravity gradient tensor δ T zz res ), especially for the Qaidam, Sichuan, Tarim, and Turpan basins. Numerical results over the Himalayan region demonstrate that the GC component δ T z res has some potential in geophysical inversion. These residual GC functionals would help to get a better knowledge of the internal structures of the Earth and other planetary objects.

Advances in the modeling of the Iberian thermal lithosphere and perspectives on deep geothermal studies

Renewable energy sources are key to achieve the transition toward clean energy system. Among them, the geothermal energy has a production whose effectiveness requires sufficient understanding of the temperature distribution and fluid circulation at depth, as well as of the lithological and petrophysical properties of the crust. The focus of this paper is twofold: first, we summarize the main advances in the development of new methodologies and numerical codes to characterize the properties of the thermal lithosphere in terms of its, temperature, density and composition; second, based on the compilation of available thermal modelling results, we present the depth of the thermal Lithosphere–Asthenosphere Boundary (LAB) of the Iberian Peninsula and the temperature distribution at crustal depths of 5, 10, and 20 km, in addition to at Moho level. At 5 km depth, the temperature is above 110 °C with local anomalies (> 130 °C) located in the Iberian Massif and Cenozoic volcanic provinces. A similar pattern is observed at 10 and 20 km depth, where temperatures are above 190 °C and 350 °C, respectively. At 20 km depth, anomalies above > 500 °C, delineate the SE and NE Cenozoic volcanic provinces. At Moho depths, temperature ranges from 450 to 800 °C with hot regions mainly located along the Iberian Massif and the SE and NE volcanic provinces. The compiled results do not show any lithospheric anomaly that could give rise to high temperatures at shallow depths, but they do show an acceptable exploitation potential at intermediate depths. With regard to the direct use of district and greenhouse heating and for industrial processes, the potential is great throughout the Peninsula, the main challenges being the availability of groundwater and drilling costs.

Thermodynamic constraints on the petrogenesis of massif-type anorthosites and their parental magmas

Development of computational modeling tools has revolutionized studies of magmatic processes over the last four decades. Their refinement from binary mixing equations to thermodynamically controlled geochemical assimilation models has provided more comprehensive and detailed modeling constraints of an array of magmatic systems. One of the questions that has not yet been vigorously studied using thermodynamic constraints is the origin of massif-type anorthosites. The parental melts to these intrusions are hypothesized to be either mantle-derived high-Al basaltic melts that undergo crustal contamination or monzodioritic melts derived directly from lower crust. On the other hand, many studies suggest that the monzodioritic rocks do not represent parental melts but instead represent crystal remnants of residual liquids left after crystal fractionation of parental melts. Regardless of the source or composition, magmas that produce massif-type anorthosites have been suggested to have undergone polybaric (~1000–100 MPa) fractional crystallization while ascending through the lithosphere. We conducted lower crustal melting, assimilation-fractional crystallization, and isobaric and polybaric fractional crystallization major element modeling using two thermodynamically constrained modeling tools, the Magma Chamber Simulator (MCS) and rhyolite-MELTS, to test the suitability of these tools and to study the petrogenesis of massif-type anorthosites. Comparison of our models with a large suite of whole-rock data suggests that the massif-type anorthosite parental melts were high-Al basalts that were produced when hot mantle-derived partial melts assimilated lower crustal material at Moho levels. These contaminated basaltic parental magmas then experienced polybaric fractional crystallization at different crustal levels (~40 to 5 km) producing residual melts that crystallized as monzodioritic rocks. Model outcomes also support the suggestion that the cumulates produced during polybaric fractional crystallization likely underwent density separation, thus producing the plagioclase-rich anorthositic rocks. The modeled processes are linked to a four-stage model that describes the key petrogenetic processes that generate massif-type anorthosites. The presented framework enables further detailed thermodynamic and geochemical modeling of individual anorthosite intrusions using MCS and involving trace element and isotope constrains. • Thermodynamic constraints on the origin of massif-type anorthosites using MCS. • Models provide indications for the composition and source of parental magmas. • The parental magmas were generated by AFC processes of mantle-derived melts. • The models are linked to general magmatic processes during anorthosite formation. • The general models set the foundation for detailed modeling of individual intrusions.

Sub-axial deformation in oceanic lower crust: Insights from seismic reflection profiles in the Enderby Basin and comparison with the Oman ophiolite

We analyzed high-quality seismic reflection profiles across the ocean-continent transition in the Enderby Basin between the Kerguelen Plateau and the Antarctic margin. There, we observe numerous high-amplitude dipping reflections in the lower oceanic crust which was accreted at a magmatic spreading center as testified by the almost uniform 6.4-7 km thick crust and its unfaulted, flat top basement. The deep reflections are rooting onto the Moho and are dipping both ridgeward and continentward. They occur in dense networks in mature oceanic crust as well as close to the continentward termination of oceanic crust and in the ocean-continent transition zone. The comparison with field observations in the Oman ophiolite suggests that these lower crustal dipping reflectors could correspond to syn-magmatic faults. In Oman, very high temperature (up to syn-magmatic), high temperature (sub-solidus plastic deformation) and low temperature (brittle) deformation coexist along the same fault over distances of a few hundred meters at Moho level. This very high temperature gradient may be explained by the sudden and intense interaction between crystallizing magmas and hydrothermal fluids induced by the episodic nucleation of faults in a context of continuous magmatic spreading. The igneous layering becomes extremely irregular compared to its monotonous sub-horizontal orientation away from the faults which, together with enhanced hydrothermal alteration restricted to the fault zones, might change the physical properties (velocity, density) and increase the reflectivity of syn-magmatic faults. We further speculate that these processes could explain the brightness of the lower crustal dipping reflectors observed in our seismic reflection data. Both the seismic reflection profiles of the Enderby Basin and the Oman ophiolite show evidence for syn-accretion tectonism at depth together with the systematic rotation of originally horizontal lava flows or originally vertical dikes, pre-dating cessation of magmatic activity. This indicates ubiquitous deformation processes within the axial zone of magmatic spreading centers.

Гипербазиты как фактор геодинамики по результатам исследований на Таймырском геофизическом полигоне

Seismic data indicate the widespread inversion origin of the largest geological structures of the Taimyr Peninsula and record repeated changes of the direction of tectonic movements. The inversion the Yenisei-Khatanga trough led to the formation of a system of megaarchs over the ancient depocenter containing massive hyperbasite intrusions at the Moho level, which are marked by the gravity maximum. These intrusive processes are an important factor that can initiate inversion and contribute significantly to its development under permanent compressional tectonic regime in the region.According to the geophysical data, the development of troughs in the north of Central and Western Siberia is associated with the formation of eclogites under the depocenters, at the Moho level. Consistent with the electrical resistivity anomalies, eclogites are plastic and overlap mantle hyperbasites of lower density that leads to gravitational buoying up of ultramafic formations. This, along with metamorphic and metasomatic processes, acts as an impulse for the uplift in the axial parts of the troughs. Simultaneously, expansion and acceleration of sagging, due to the displacement of plastic eclogites and crystal-mantle mixture from the penetration zone towards the peripheral parts of the sedimentary basin, result in the uncompensated sediment accumulation and the formation of the Neocomian clinoform complex.

Textural Insights on Significance of Ophiolitic Chromitites, with Special Reference to Oman

AbstractBased on the textural and crystallographic study of four chromitite sites in the Oman ophiolite, we show that chromite crystallized in situ at Moho transition zone, where exposed as podiform chromitite deposits.Crystallization operated either by crystal fractionation in a mini‐magma chamber (Tuf dyke), or more commonly by melt‐rock reaction of a hydrated Cr‐rich melt and enclosing dunite. Oxidizing conditions at Moho level triggered the crystallization of chromite at the expense of the corroded olivine network. High‐temperature mantle flow (1100–1200 °C), recorded in the joined reacting olivine aggregates, constrains the timing of chromitite formation.Models of genesis of chromitite deposits must account for a hydrous component initiating partial melting of refractory peridotite, and the revealed occurrence of ultra‐high‐pressure cratonic phases included in some chromite crystals of some ophiolitic chromitites.

Hydrated Peridotite–Basaltic Melt Interaction Part II: Fast Assimilation of Serpentinized Mantle by Basaltic Magma

The most abundant terrestrial lavas, mid-ocean ridge and ocean island basalt (MORB and OIB), are commonly considered to be derived from a depleted MORB-mantle component and more specific, variably enriched mantle plume sources. However, findings of oceanic lavas and mafic cumulates issued from melts, enriched in chlorine and having a radiogenic Sr ratio, can be attributed to an interaction between the asthenosphere-derived melts and lithospheric peridotite variably hydrated due to penetration of hydrothermal water down to and below Moho level. To constrain mechanisms and rates responsible for the interaction, we report results of experiments of reaction between serpentinite and tholeiitic basaltic melt. Results show that the reaction proceeds via a multi-stage mechanism: (i) transformation of serpentinite into Cr-rich spinel-bearing harzburgite containing pore fluid, (ii) partial melting and dissolution of the harzburgite assemblage with formation of interstitial hydrous melts, and (iii) final assimilation of the Cr-rich spinel-bearing harzburgite/dunite and formation of hybrid basaltic melts with high MgO and elevated Cr and Ni contents. Assimilation of serpentinite by basaltic melt may occur under elevated melt/rock ratios and may lead to chromitite formation. Our experiments provide evidence that MORB and high-Mg-Cr orthopyroxene-rich cumulates depleted in incompatible elements can be produced from common mid-ocean ridge basaltic melts modified by reaction with hydrated lithospheric peridotite. We established that the rate of assimilation of serpentinized peridotite is controlled by silica diffusion in the reacting hydrous basaltic melt.

Melt hybridization and metasomatism triggered by syn-magmatic faults within the Oman ophiolite: A clue to understand the genesis of the dunitic mantle-crust transition zone

On Earth, most of the critical processes happen at the frontiers between envelopes and especially at the Moho between the mantle and the crust. Beneath oceanic spreading centers, the dunitic transition zone (DTZ) appears as a major interface between the upwelling and partially molten peridotitic mantle and the accreting gabbroic lower crust. Better constraints on the processes taking part in the DTZ allows improved understanding of the interactions between silicate melts and hydrated fluids, which act competitively to generate the petrological Moho. Here we combine mineral and whole rock major and trace element data with a structural approach along three cross-sections up to 300 m thick above the fossil Maqsad mantle diapir (Oman ophiolite) in order to understand the vertical organization of the DTZ with depth. Our results highlight that most of the faults or fractures cross-cutting the DTZ were ridge-related and active at an early, high temperature magmatic stage. Chemical variations along the cross-sections define trends with a characteristic vertical scale of few tens of meters. There is a clear correlation between the chemical variation pattern and the distribution of fault zones, not only for fluid-mobile elements but also for immobile elements such as REE and HFSE. Faults, despite displaying very limited displacements, enhanced both melt migration and extraction up to the crust and deep hydrothermal fluids introduction down to the Moho level. We propose that these faults are a vector for upwelling melt modification by hybridization, with hydrothermal fluids and/or silicic hydrous melts, and crystallization. Infiltration of these melts or fluids in the country rock governs part of the gradational evolutions recorded in composition of both the olivine matrix and interstitial phases away from faults. Finally, these faults likely control the thermal structure of the mantle-crust transition as evidenced by the spatial distribution of the crystallization products from percolating melts, organizing the transition zone into pure dunites to impregnated dunites horizons. In this context, the DTZ appears as a reactive interface that developed by the combination of three primary processes: tectonics, magmatism and deep, high temperature hydrothermal circulations. Accordingly, these features fundamentally contribute to the variable petrological and geochemical organization of the DTZ and possibly of the lower crust below oceanic spreading centers, and may be a clue to interpret part the heterogeneity observed in MORB signatures worldwide.

Extreme geochemical variability through the dunitic transition zone of the Oman ophiolite: Implications for melt/fluid-rock reactions at Moho level beneath oceanic spreading centers

The Maqsad area in the Oman ophiolite exposes a >300 m thick dunitic mantle-crust transition zone (DTZ) that developed above a mantle diapir. The Maqsad DTZ is primarily made of “pure” dunites (olivine with scattered chromite and chromite seams) and “impregnated” dunites, which exhibit a significant lithological variability, including various kinds of clinopyroxene-, plagioclase-, orthopyroxene-, amphibole (hornblende/pargasite)-bearing dunites. These minerals are interstitial between olivine grains and their variable abundance and distribution suggest that they crystallized from a percolating melt. Generally studied through in-situ mineral characterization, the whole rock composition of dunites is poorly documented. This study reports on whole rock and minerals major and trace element contents on 79 pure to variably impregnated dunites collected systematically along cross sections from the base to the top of the DTZ. In spite of its high degree of depletion, the olivine matrix is selectively enriched in the most incompatible trace elements such as LREE, HFSE, Th, U, Rb and Ba. These data support the view that this enrichment has been acquired early in the magmatic evolution of the DTZ, during the dunitization process itself. The dissolution of orthopyroxene from mantle harzburgites enhanced by the involvement of hydrothermal fluids produced low amounts of melts enriched in silica and in some trace elements that re-equilibrated with the olivine matrix. This pristine signature of the DTZ dunite was eventually variably altered by percolation of melts with a Mid-Ocean Ridge Basalt (MORB) affinity but displaying a wide spectrum of composition attributable to evolution by fractional crystallization and hybridization with the silica enriched, hydrated melts. The olivine matrix has been partially or fully re-equilibrated with these melts, smoothing the early strong concave-upward REE pattern in dunite. The chemical variability in the interstitial minerals bears witness of the percolation of MORB, issued from the mantle decompression melting, variably hybridized with melt batches produced within the DTZ by melt-rock reaction and poorly homogenized before reaching the lower crust. Our results lead to the conclusion that pure and impregnated dunites are end-members that recorded different stages of the same initial igneous processes: pure dunites are residues left after extraction of a percolating melt while impregnated dunites correspond to a stage frozen before complete melt extraction. Therefore dunites trace elements contents allow deciphering the multi-stage processes that led to their formation at the mantle-crust transition zone.

GEODYNAMICS OF THE TAIMYR PENINSULA FROM GEOPHYSICAL DATA

Recently, a unique geophysical polygon has been created on the Taimyr Peninsula and the adjacent territories. Its densely spaced set of seismic and electric prospecting profiles makes it possible to investigate the structure of the continental lithosphere to the depths of 50–60 km (Fig. 1). The sedimentary cover and the deep horizons of the crust and upper mantle are studied in detail to discover new facts in the history of the geological development of the northern Siberian platform. The contrasting anomalies recorded at the lower crust levels give evidence of considerable changes in density and electrical conductivity, which coincide in plan with the most significant geological structures in the northern regions of Central and West Siberia (Fig. 2). The relationships of the largest uplifts and troughs with the decomposition and compacting processes at the level of Moho are indicative of, on the one hand, the metamorphism occurring under the influence of asthenospheric fluids, and, on the other, the determining role of lithostatic pressure equalization in the formation of these structures. Thus, for the first time, the direct geophysical surveys have shown that the nature of tectonic processes is not limited to only simple horizontal movements of the cratons and associated deformation. Our data shows that the deep geological structure of the study area can be explained, in addition to collision, by the vertical stresses associated with the compaction/decompaction of the crust under the influence of the materials uplifted from the mantle and the lithostatic pressure equalization compensating for the density changes. The vertical tectonic movements are not unidirectional and periodically change their sign, as evidenced by the inversion nature of the largest structures of the sedimentary cover in the Taimyr Peninsula (Fig. 3). In the course of the geological development, the troughs are generally replaced by banks, and uplifts and ridges are involved in subduction, to varying degrees. According to the data obtained, such changes are determined by the differences in the depth levels of the large blocks of the lower crust and upper mantle. These variations seem to play a significant role on other continents; in that regard, their further studies will contribute to a better understanding of global geodynamics.

Can high‐temperature, high‐heat flux hydrothermal vent fields be explained by thermal convection in the lower crust along fast‐spreading Mid‐Ocean Ridges?

AbstractWe present numerical models to explore possible couplings along the axis of fast‐spreading ridges, between hydrothermal convection in the upper crust and magmatic flow in the lower crust. In an end‐member category of models corresponding to effective viscosities μM lower than 1013 Pa.s in a melt‐rich lower crustal along‐axis corridor and permeability k not exceeding ∼10−16 m2 in the upper crust, the hot, melt‐rich, gabbroic lower crust convects as a viscous fluid, with convection rolls parallel to the ridge axis. In these models, we show that the magmatic‐hydrothermal interface settles at realistic depths for fast ridges, i.e., 1–2 km below seafloor. Convection cells in both horizons are strongly coupled and kilometer‐wide hydrothermal upflows/plumes, spaced by 8–10 km, arise on top of the magmatic upflows. Such magmatic‐hydrothermal convective couplings may explain the distribution of vent fields along the East (EPR) and South‐East Pacific Rise (SEPR). The lower crustal plumes deliver melt locally at the top of the magmatic horizon possibly explaining the observed distribution of melt‐rich regions/pockets in the axial melt lenses of EPR and SEPR. Crystallization of this melt provides the necessary latent heat to sustain permanent ∼100 MW vents fields. Our models also contribute to current discussions on how the lower crust forms at fast ridges: they provide a possible mechanism for focused transport of melt‐rich crystal mushes from moho level to the axial melt lens where they further crystallize, feed eruptions, and are transported both along and off‐axis to produce the lower crust.

Locating the depth of magma supply for volcanic eruptions, insights from Mt. Cameroon.

Mt. Cameroon is one of the most active volcanoes in Africa and poses a possible threat to about half a million people in the area, yet knowledge of the volcano’s underlying magma supply system is sparse. To characterize Mt. Cameroon’s magma plumbing system, we employed mineral-melt equilibrium thermobarometry on the products of the volcano’s two most recent eruptions of 1999 and 2000. Our results suggest pre-eruptive magma storage between 20 and 39 km beneath Mt. Cameroon, which corresponds to the Moho level and below. Additionally, the 1999 eruption products reveal several shallow magma pockets between 3 and 12 km depth, which are not detected in the 2000 lavas. This implies that small-volume magma batches actively migrate through the plumbing system during repose intervals. Evolving and migrating magma parcels potentially cause temporary unrest and short-lived explosive outbursts, and may be remobilized during major eruptions that are fed from sub-Moho magma reservoirs.

The magmatic system beneath the Tristan da Cunha Island: Insights from thermobarometry, melting models and geophysics

This study provides new insights on the conditions of melt generation and of magma transport and storage beneath Tristan da Cunha Island in the South Atlantic. Situated at the seaward end of the Walvis Ridge-guyot hotspot track, this island is related to the evolving magmatic system of the Tristan plume. Much is known about the geochemical and isotopic composition of the alkaline lavas on Tristan, but the pressure–temperature conditions of the hotspot magmas are under-explored. This contribution reports new data from a suite of 10 samples collected during a geologic–geophysical expedition in 2012. The focus of this study is on the least-evolved, phenocryst-rich basanite lavas but we also included a sample of trachyandesite lava erupted in 1961. Mineral-melt equilibrium thermobarometry uses the composition of olivine, clinopyroxene and plagioclase phenocrysts. In addition to bulk-rock data we also analysed olivine-hosted melt inclusions for the P–T calculations. The results for olivine-melt and clinopyroxene-melt calculations suggest crystallization conditions of around 1200–1250°C and 0.8–1.3GPa for the least-evolved magmas (ankaramitic basanites). Combined with seismological evidence for a Moho depth of about 19km, these results imply magma storage and partial crystallization of Tristan magmas in the uppermost mantle and at Moho level. The trachyandesite yielded values of about 1000°C and 0.2–0.3GPa (6 to 10km depth), indicating further crystallization within the crust.Constraints on the depth and degree of melting at the source of Tristan basanites were derived from REE inverse modelling using our new trace element data. The model predicts 5% melt generation from a melting column with its base at 80–100km and a top at 60km, which is consistent with the lithospheric thickness resulting from cooling models and seismological observations. The thermobarometry and melting models combined suggest a mantle potential temperature of about 1360°C for the Tristan hotspot.

Structural contribution from the Oman ophiolite to processes of crustal accretion at the East Pacific Rise

AbstractA comprehensive model for the activity of the elementary accretion segment at fast‐spreading ridges relies on integration of structural data from the Oman ophiolite and geophysical results from the East Pacific Rise (EPR) around 9°N, which are of comparable size and spreading rates. The axial melt lens at shallow crustal level provides a link between Deval segmentation at the seafloor and a lower melt sill at Moho level, imaged at the EPR as a crustal melt zone (CMZ) and mapped in Oman as the Moho transition zone (MTZ). Both are attached to a mantle upwelling at the EPR, and to a frozen diapir in Oman. The physical link between diapiric mantle uprising at the Moho and Devals segmentation at the seafloor is the melt being injected from the mantle into the lower MTZ, ponding there, and then being released by powerful injections into the upper melt lens. The magma chamber covers the diapir at a distance of 5 km from the ridge axis.

Segmentation of plate coupling, fate of subduction fluids, and modes of arc magmatism in Cascadia, inferred from magnetotelluric resistivity

AbstractFive magnetotelluric (MT) profiles have been acquired across the Cascadia subduction system and transformed using 2‐D and 3‐D nonlinear inversion to yield electrical resistivity cross sections to depths of ∼200 km. Distinct changes in plate coupling, subduction fluid evolution, and modes of arc magmatism along the length of Cascadia are clearly expressed in the resistivity structure. Relatively high resistivities under the coasts of northern and southern Cascadia correlate with elevated degrees of inferred plate locking, and suggest fluid‐ and sediment‐deficient conditions. In contrast, the north‐central Oregon coastal structure is quite conductive from the plate interface to shallow depths offshore, correlating with poor plate locking and the possible presence of subducted sediments. Low‐resistivity fluidized zones develop at slab depths of 35–40 km starting ∼100 km west of the arc on all profiles, and are interpreted to represent prograde metamorphic fluid release from the subducting slab. The fluids rise to forearc Moho levels, and sometimes shallower, as the arc is approached. The zones begin close to clusters of low‐frequency earthquakes, suggesting fluid controls on the transition to steady sliding. Under the northern and southern Cascadia arc segments, low upper mantle resistivities are consistent with flux melting above the slab plus possible deep convective backarc upwelling toward the arc. In central Cascadia, extensional deformation is interpreted to segregate upper mantle melts leading to underplating and low resistivities at Moho to lower crustal levels below the arc and nearby backarc. The low‐ to high‐temperature mantle wedge transition lies slightly trenchward of the arc.

Present-day 3D structural model of the Po Valley basin, Northern Italy

A 3D structural model of the Po Valley basin (Northern Italy) was built by integrating the dataset available from the public domain (DEM, wells, isobath-maps, cross-sections, outcrop-trends).The model shows the complex foredeep-foreland architecture across the basin, from the Moho level to the topography while illustrating the top Basement, top Triassic, top Mesozoic and base-Pliocene surface-grid structures.The results, by model slicing and isopach-map reconstruction, suggest that the deep Moho architecture and the original tectonics of the ancient Adria-Po Valley passive continental margin are key factors in controlling the current structures type, orientation and distribution, at any of the shallowest levels across the basin. In particular, the analysis of the final 3D Mesozoic geometries against the pre and post-Alpine trends confirms the structural interference between the mutually perpendicular Triassic–Jurassic extensional structures and the Tertiary compressional ones, this being evident from the regional to the oil-field scale.Despite the model uncertainty, mainly related to its dimension versus the original non-homogeneous dataset quality and distribution, the final geo-volume offers, for the first time in the region, a continuous three-dimensional visualization of the Po Valley tectonic architecture. It provides, simultaneously, a powerful tool for the reviewing of the basin structures and the potential support to future applications for both industry and academia.

Deep electrical resistivity structure of the northwestern U.S. derived from 3-D inversion of USArray magnetotelluric data

Long period (10–20,000 s) magnetotelluric (MT) data are being acquired across the continental USA on a quasi-regular grid of ∼70 km spacing as an electromagnetic component of the National Science Foundation EarthScope/USArray Program. These data are sensitive to fluids, melts, and other orogenic indicators, and thus provide a valuable complement to other components of EarthScope. We present and interpret results of 3-D MT data inversion from 325 sites acquired from 2006–2011 to provide a regional scale view of electrical resistivity from the middle crust to nearly the mantle transition zone, covering an area from NW Washington to NW Colorado. Beneath the active extensional subprovinces in the south-central region, on average we see a resistive upper crust, and then extensive areas of low resistivity in the lower crust and uppermost mantle. Further below, much of the upper half of the upper mantle appears moderately resistive, then subsequently the lower upper mantle becomes moderately conductive. This column suggests a dynamic process of moderately hydrated and fertile deeper upper mantle upwelling during extension, intersection of that material with the damp solidus causing dehydration and melting, and upward exodus of generated mafic melts to pond and exsolve saline fluids near Moho levels. Lithosphere here is very thin. To the east and northeast, thick sections of resistive lithosphere are imaged under the Wyoming and Medicine Hat Cratons. These are punctuated with numerous electrically conductive sutures presumably containing graphitic or sulfide-bearing meta-sediments deeply underthrust and emplaced during ancient collisions. Below Cascadia, the subducting Juan de Fuca and Gorda lithosphere appears highly resistive. Suspected oceanic lithosphere relicts in the central NW part of the model domain also are resistive, including the accreted “Siletzia” terrane beneath the Coast Ranges and Columbia Embayment, and the seismically fast “slab curtain” beneath eastern Idaho interpreted by others as stranded Farallon plate. Upwelling of deep fluid or melt in the Cascade volcanic arc region manifests as conductive features at several scales. These include quasi-horizontal conductive patches under the arc and fore-arc, likely denoting fluids evolved via breakdown of hydrous minerals in the current down-going slab. In the backarc, low resistivities concentrate in “plumes” connecting into a deeper aesthenospheric layer to the east, consistent with subduction-driven upwelling of hot, hydrated or melted, aesthenospheric mantle. Low resistivities (<10 Ωm) deep beneath the stable cratons suggest higher levels of hydration there, and/or influence of poorly resolved structures outside the array.

Combining controlled-source seismology and local earthquake tomography to derive a 3-D crustal model of the western Alpine region

SUMMARY We present a newly developed approach of combining controlled-source seismology (CSS) and local earthquake tomography (LET) data to obtain a new 3-D crustal model of the western Alpine region. Our approach combines either data by taking into account the strengths of the individual seismic methods. Our western Alpine 3-D model is primarily based on a welldefined Moho, constrained by CSS and LET data, and includes smooth lateral variations in seismic velocities mainly constrained by LET data, but locally also by CSS data. The consistent combination of results from the two different seismic methods is feasible due to LET Moho elements, as defined by characteristic P-wave velocities and their uncertainty estimates. These uncertainty estimates are based on values of the diagonal element of the resolution matrix, absolute P-wave velocities that are typical for crust and mantle and a specific velocity gradient across the Moho discontinuity. Finally, our definition of LET Moho elements and their uncertainties is validated by comparisons of highest quality Moho results from both methods coinciding in 353 localities. Our model clearly shows three Moho surfaces, being Europe, Adria and Liguria as well as major tectonic structures like suture zones and the high-velocity Ivrea body. In general, it is in a good agreement with previous studies. The biggest differences occur along plate boundaries, where the strong lateral velocity variations arebestresolvedbyLET.DuetothelargernumberofavailableMohoreflectorelementsamore accurate definition of plate boundaries at Moho level is possible and, therefore, new insights in deep lithosphere structures of the Alpine collision zone can be expected. Furthermore, our new 3-D crustal model directly includes a 3-D migrated image of the Ivrea body.

A New View on the Petrogenesis of the Oman Ophiolite Chromitites from Microanalyses of Chromite-hosted Inclusions

To decipher the petrogenesis of chromitites from the Moho Transition Zone of the Cretaceous Oman ophiolite, we carried out detailed scanning electron microscope and electron microprobe investigations of ∼500 silicate and chromite inclusions and their chromite hosts, and oxygen isotope measurements of seven chromite and olivine fractions from nodular, disseminated, and stratiform ore bodies and associated host dunites of the Maqsad area, Southern Oman. The results, coupled with laboratory homogenization experiments, allow several multiphase and microcrystal types of the chromite-hosted inclusions to be distinguished. The multiphase inclusions are composed of micron-size (1–50 μm) silicates (with rare sulphides) entrapped in high cr-number [100Cr/(Cr + Al) up to 80] chromite. The high cr-number chromite coronas and inclusions are reduced (oxygen fugacity, fO2, of ∼3 log units below the quartz–fayalite–magnetite buffer, QFM). The reduced chromites, which crystallized between 600 and 950°C at subsolidus conditions, were overgrown by more oxidized host chromite (fO2 ≈ QFM) in association with microcrystal inclusions of silicates (plagioclase An86, clinopyroxene, and pargasite) that were formed between 950 and 1050°C at 200 MPa from a hydrous hybrid mid-ocean ridge basalt (MORB) melt. Chromium concentration profiles through the chromite coronas, inclusions, and host chromites indicate non-equilibrium fractional crystallization of the chromitite system at fast cooling rates (up to ∼0·1°C a−1). Oxygen isotope compositions of the chromite grains imply involvement of a mantle protolith (e.g. serpentinite and serpentinized peridotite) altered by seawater-derived hydrothermal fluids in an oceanic setting. Our findings are consistent with a three-stage model of chromite formation involving (1) mantle protolith alteration by seawater-derived hydrothermal fluids yielding serpentinites and serpentinized harzburgites, which were probably the initial source of chromium, (2) subsolidus crystallization owing to prograde metamorphism, followed by (3) assimilation and fractional crystallization of chromite from water-saturated MORB. This study suggests that the metamorphic protolith assimilation occurring at the Moho level may dramatically affect MORB magma chemistry and lead to the formation of economic chromium deposits.

On the evolution of large ultramafic magma chambers and timescales for flood basalt eruptions

[1] Large igneous provinces are characterized by magmatic activity on two distinct timescales. While these provinces have total active lifetimes of order 10–30 Ma, most of the erupted volume is emplaced within 1 Ma in many cases. The longer timescale is consistent with plume or tectonic models for mantle melting responsible for flood volcanism, but the shorter “main stage” timescale is enigmatic. We hypothesize that cessation of main stage eruptions reflects shutoff of dike propagation from the deep crust due to the onset of thermally activated creep on a ∼1 Ma timescale, with intrusive processes and minor eruptions continuing over 10–30 Ma. To test this hypothesis we model deep magma differentiation and the stability of Moho level magma reservoirs. Assuming mantle volatile contents, fractionation results in concentration and deep exsolution of CO2, with the geothermal gradient and melt influx setting the timescale for buoyancy production and thus timing between individual eruptions. Chemical evolution generally occurs rapidly compared to heat conduction from the magma body. Although the viscous response of surrounding rocks depends on lower crustal rheology, we find that thermally induced creep can reasonably prevent dike propagation within 1 Ma of intrusion. However, if melt influx is high or heat transfer from the magma chamber is low, viscous creep may outpace differentiation. In this regime, continued melt influx spreads along the Moho until external stresses provide a destabilizing trigger. The coevolution of large-scale melting and lower crustal rheology may thus control a progression of large igneous province emplacement from largely eruptive to largely intrusive magmatism.