A new tectonic model, postulating the growth of giant subduction-accretion complexes along a single magmatic arc now found contorted between Siberia and Baltica, shows that Asia grew by 5.3 million square kilometres during the Palaeozoic era. Half of this growth may have occurred by the addition of juvenile crust newly extracted from the mantle, supporting models of considerable continental growth continuing throughout the Phanerozoic eon.

Evolution of the Altaid tectonic collage and Palaeozoic crustal growth in Eurasia

Geophysical, petrological, and geochemical data provide important clues about the composition of the deep continental crust. On the basis of seismic refraction data, we divide the crust into type sections associated with different tectonic provinces. Each shows a three-layer crust consisting of upper, middle, and lower crust, in which P wave velocities increase progressively with depth. There is large variation in average P wave velocity of the lower crust between different type sections, but in general, lower crustal velocities are high (>6.9 km s−1) and average middle crustal velocities range between 6.3 and 6.7 km s−1. Heat-producing elements decrease with depth in the crust owing to their depletion in felsic rocks caused by granulite facies metamorphism and an increase in the proportion of mafic rocks with depth. Studies of crustal cross sections show that in Archean regions, 50–85% of the heat flowing from the surface of the Earth is generated within the crust. Granulite terrains that experienced isobaric cooling are representative of middle or lower crust and have higher proportions of mafic rocks than do granulite terrains that experienced isothermal decompression. The latter are probably not representative of the deep crust but are merely upper crustal rocks that have been through an orogenic cycle. Granulite xenoliths provide some of the deepest samples of the continental crust and are composed largely of mafic rock types. Ultrasonic velocity measurements for a wide variety of deep crustal rocks provide a link between crustal velocity and lithology. Meta-igneous felsic, intermediate and mafic granulite, and amphibolite facies rocks are distinguishable on the basis of P and S wave velocities, but metamorphosed shales (metapelites) have velocities that overlap the complete velocity range displayed by the meta-igneous lithologies. The high heat production of metapelites, coupled with their generally limited volumetric extent in granulite terrains and xenoliths, suggests they constitute only a small proportion of the lower crust. Using average P wave velocities derived from the crustal type sections, the estimated areal extent of each type of crust, and the average compositions of different types of granulites, we estimate the average lower and middle crust composition. The lower crust is composed of rocks in the granulite facies and is lithologically heterogeneous. Its average composition is mafic, approaching that of a primitive mantle-derived basalt, but it may range to intermediate bulk compositions in some regions. The middle crust is composed of rocks in the amphibolite facies and is intermediate in bulk composition, containing significant K, Th, and U contents. Average continental crust is intermediate in composition and contains a significant proportion of the bulk silicate Earth's incompatible trace element budget (35–55% of Rb, Ba, K, Pb, Th, and U).

Nature and composition of the continental crust: A lower crustal perspective

When continents rift to form new ocean basins, the rifting is sometimes accompanied by massive igneous activity. We show that the production of magmatically active rifted margins and the effusion of flood basalts onto the adjacent continents can be explained by a simple model of rifting above a thermal anomaly in the underlying mantle. The igneous rocks are generated by decompression melting of hot asthenospheric mantle as it rises passively beneath the stretched and thinned lithosphere. Mantle plumes generate regions beneath the lithosphere typically 2000 km in diameter with temperatures raised 100–200°C above normal. These relatively small mantle temperature increases are sufficient to cause the generation of huge quantities of melt by decompression: an increase of 100°C above normal doubles the amount of melt whilst a 200°C increase can quadruple it. In the first part of this paper we develop our model to predict the effects of melt generation for varying amounts of stretching with a range of mantle temperatures. The melt generated by decompression migrates rapidly upward, until it is either extruded as basalt flows or intruded into or beneath the crust. Addition of large quantities of new igneous rock to the crust considerably modifies the subsidence in rifted regions. Stretching by a factor of 5 above normal temperature mantle produces immediate subsidence of more than 2 km in order to maintain isostatic equilibrium. If the mantle is 150°C or more hotter than normal, the same amount of stretching results in uplift above sea level. Melt generated from abnormally hot mantle is more magnesian rich than that produced from normal temperature mantle. This causes an increase in seismic velocity of the igneous rocks emplaced in the crust, from typically 6.8 km/s for normal mantle temperatures to 7.2 km/s or higher. There is a concomitant density increase. In the second part of the paper we review volcanic continental margins and flood basalt provinces globally and show that they are always related to the thermal anomaly created by a nearby mantle plume. Our model of melt generation in passively upwelling mantle beneath rifting continental lithosphere can explain all the major rift-related igneous provinces. These include the Tertiary igneous provinces of Britain and Greenland and the associated volcanic continental margins caused by opening of the North Atlantic in the presence of the Iceland plume; the Parana and parts of the Karoo flood basalts together with volcanic continental margins generated when the South Atlantic opened; the Deccan flood basalts of India and the Seychelles-Saya da Malha volcanic province created when the Seychelles split off India above the Reunion hot spot; the Ethiopian and Yemen Traps created by rifting of the Red Sea and Gulf of Aden region above the Afar hot spot; and the oldest and probably originally the largest flood basalt province of the Karoo produced when Gondwana split apart. New continental splits do not always occur above thermal anomalies in the mantle caused by plumes, but when they do, huge quantities of igneous material are added to the continental crust. This is an important method of increasing the volume of the continental crust through geologic time.

Magmatism at rift zones: The generation of volcanic continental margins and flood basalts

Seismic techniques provide the highest-resolution measurements of the structure of the crust and have been conducted on a worldwide basis. We summarize the structure of the continental crust based on the results of seismic refraction profiles and infer crustal composition as a function of depth by comparing these results with high-pressure laboratory measurements of seismic velocity for a wide range of rocks that are commonly found in the crust. The thickness and velocity structure of the crust are well correlated with tectonic province, with extended crust showing an average thickness of 30.5 km and orogens an average of 46.3 km. Shields and platforms have an average crustal thickness nearly equal to the global average. We have corrected for the nonuniform geographical distribution of seismic refraction profiles by estimating the global area of each major crustal type. The weighted average crustal thickness based on these values is 41.1 km. This value is 10% to 20% greater than previous estimates which underrepresented shields, platforms, and orogens. The average compressional wave velocity of the crust is 6.45 km/s, and the average velocity of the uppermost mantle (Pn velocity) is 8.09 km/s. We summarize the velocity structure of the crust at 5-km depth intervals, both in the form of histograms and as an average velocity-depth curve, and compare these determinations with new measurements of compressional wave velocities and densities of over 3000 igneous and metamorphic rock cores made to confining pressures of 1 GPa. On the basis of petrographic studies and chemical analyses, the rocks have been classified into 29 groups. Average velocities, densities, and standard deviations are presented for each group at 5-km depth intervals to crustal depths of 50 km along three different geotherms. This allows us to develop a model for the composition of the continental crust. Velocities in the upper continental crust are matched by velocities of a large number of lithologies, including many low-grade metamorphic rocks and relatively silicic gneisses of amphibolite facies grade. In midcrustal regions, velocity gradients appear to originate from an increase in metamorphic grade, as well as a decrease in silica content. Tonalitic gneiss, granitic gneiss, and amphibolite are abundant midcrustal lithologies. Anisotropy due to preferred mineral orientation is likely to be significant in upper and midcrustal regions. The bulk of the lower continental crust is chemically equivalent to gabbro, with velocities in agreement with laboratory measurements of mafic granulite. Garnet becomes increasingly abundant with depth, and mafic garnet granulite is the dominant rock type immediately above the Mohorovicic discontinuity. Average compressional wave velocities of common crustal rock types show excellent correlations with density. The mean crustal density calculated from our model is 2830 kg/m3, and the average SiO2 content is 61.8%.

Seismic velocity structure and composition of the continental crust: A global view

Preface 1. Basic tools 2. Elasticity and Hooke's law 3. Seismic wave propagation 4. Effective media 5. Granular media 6. Fluid effects on wave propagation 7. Empirical relations 8. Flow and diffusion 9. Electrical properties Appendices.

/pdf/the-rock-physics-handbook-tools-for-seismic-analysis-of-58lw74s1oi.pdf

The Rock Physics Handbook: Tools for Seismic Analysis of Porous Media

A compilation of compressional-wave ( V p) and shear-wave ( V s) velocities and densities for a wide variety of common lithologies is used to define new nonlinear, multivalued, and quantitative relations between these properties for the Earth's crust. Wireline borehole logs, vertical seismic profiles, laboratory measurements, and seismic tomography models provide a diverse dataset for deriving empirical relations between crustal V p and V s. The proposed V s as a function of V p relations fit V s and V p borehole logs in Quaternary alluvium and Salinian granites as well as laboratory measurements over a 7-km/sec-wide range in V p. The relations derived here are very close to those used to develop a regional 3D velocity model for southern California, based on pre-1970 data, and thus provide support for that model. These data, and these relations, show a rapid increase in V s as V p increases to 3.5 km/sec leading to higher shear-wave velocities in young sedimentary deposits than commonly assumed. These relations, appropriate for active continental margins where earthquakes are prone to occur, suggests that amplification of strong ground motions by shallow geologic deposits may not be as large as predicted by some earlier models.

Empirical relations between elastic wavespeeds and density in the Earth's crust

A reflection observed on multi-channel seismic profiles along and across the East Pacific Rise between 8°50′ N and 13°30′ N is interpreted to arise from the top of a crustal magma chamber located 1.2–2.4 km below the sea floor. The magma chamber is quite narrow (<4 – 6 km wide), but can be traced as a nearly continuous feature for tens of kilometres along the rise axis.

Multi-channel seismic imaging of a crustal magma chamber along the East Pacific Rise

We analyze four expanded spread profiles acquired at distances of 0, 2.1, 3.1, and 10 km (0–0.2 m.y.) from the axis of the East Pacific Rise between 9° and 10°N. Velocity-depth models for these profiles have been obtained by travel time inversion in the τ-p domain, and by x-t forward modeling using the WKBJ and the reflectivity methods. We observe refracted arrivals that allow us to determine directly the uppermost crustal velocity structure (layer 2A). At the seafloor we find very low Vp and VS/Vp values around 2.2 km/s and ≤ 0.43. In the topmost 100–200 m of the crust, Vp remains low (≤ 2.5 km/s) then rapidly increases to 5 km/s at ∼500 m below the seafloor. High attenuation values (Qp < 100) are suggested in the topmost ∼500 m of the crust. The layer 2–3 transition probably occurs within the dike unit, a few hundred meters above the dike-gabbro transition. This transition may mark the maximum depth of penetration by a cracking front and associated hydrothermal circulation in the axial region above the axial magma chamber (AMC). The on-axis profile shows arrivals that correspond to the bright AMC event seen in reflection lines within 2 km of the rise axis. The top of the AMC lies 1.6 km below the seafloor and consists of molten material where Vp ≈ 3 km/s and VS = 0. Immediately above the AMC, there is a zone of large negative velocity gradients where, on the average, Vp decreases from ∼6.3 to 3 km/s over a depth of approximately 250 m. Associated with the AMC there is a low velocity zone (LVZ) that extends to a distance no greater than 10 km away from the rise axis. At the top of the LVZ, sharp velocity contrasts are confined to within 2 km of the rise axis and are associated with molten material or material with a high percentage of melt which would be concentrated only in a thin zone at the apex of the LVZ, in the axial region where the AMC event is seen in reflection lines. Away from the axis, the transition to the LVZ is smoother, the top of the LVZ is deeper, and the LVZ is less pronounced. The bottom of the LVZ is probably located near the bottom of the crust and above the Moho. Moho arrivals are observed in the profiles at zero and at 10 km from the rise axis. Rather than a single discontinuity, these arrivals indicate an approximately 1-km-thick Moho transition zone.

The structure of 0‐ to 0.2‐m.y.‐old oceanic crust at 9°N on the East Pacific Rise from expanded spread profiles

We present the results of the analysis of expanding spread profiles (ESPs) collected on and near the axis of the East Pacific Rise at 13°N. These profiles were collected at 0, 1.1, 2.1, 3.6, and 9.5 km from the rise axis, and all but the most distant profile show a distinct low-velocity zone (LVZ) located within layer 3 of the oceanic crust. At the ridge crest, the top of the magma chamber is at the base of layer 2, while 3.6 km off axis, the roof of the LVZ is 1.1 km below the top of layer 3. The profile farthest from the ridge could possibly have a residual LVZ confined to the lower 1.5 km of the crust. The total width of the LVZ, as determined from the ESP data, is at least 6 km, and possibly much greater. This wide LVZ apparently contradicts multichannel seismic data which show cross-axis reflections from the magma chamber with a width of <5 km. We suggest that a resolution of this apparent contradiction lies in a model of the rise axis with a small and transient central magma chamber of high partial melt fraction surrounded by a much larger and permanent region of hot rock with only isolated pockets of partial melt. The ESP data are sensitive to this larger region, while the reflection data accurately map the presence or absence of the central magma chamber with its high impedance contrast. We identify the presence of a layer at the top of the oceanic crust with initial P wave velocities between 2.35 and 2.6 km/s, while the S wave velocity is estimated as being ≤0.8 km/s. The layer thickness lies between 100 and 200 m. These velocities are consistent with previous estimates for the Pacific and recent results for the Atlantic. The thickness of this layer is consistent with that of layer 2A determined from geophysical measurements at Deep Sea Drilling Project hole 504B.

Thomas M. Brocher

Papers

Empirical relations between elastic wavespeeds and density in the Earth's crust

Multi-channel seismic imaging of a crustal magma chamber along the East Pacific Rise

The structure of 0‐ to 0.2‐m.y.‐old oceanic crust at 9°N on the East Pacific Rise from expanded spread profiles

Structure of young oceanic crust at 13°N on the East Pacific Rise from expanding spread profiles

Imaging the Transition From Aleutian Subduction to Yakutat Collision in Central Alaska, With Local Earthquakes and Active Source Data