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

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

Previously proposed models for the evolution of the Tyrrhenian basin-Apenninic arc system do not seem to satisfactorily explain the dynamic relationship between extension in the Tyrrhenian and compression in the Apennines. The most important regional plate kinematic constraints that any model has to satisfy in this case are: (1) the timing of extension in the Tyrrhenian and compression in the Apennines, (2) the amount of shortening in the Apennines, (3) the amount of extension in the Tyrrhenian, and (4) Africa-Europe relative motion. The estimated contemporaneous (post-middle Miocene) amounts of extension in the Tyrrhenian and of shortening in the Apennines appear to be very similar. The extension in the Tyrrhenian Sea is mostly accomplished in an E-W direction, and cannot be straightforwardly related to the calculated N-S Africa-Europe convergence. A model of outward arc migration fits all these constraints. In a subducting system, the subduction zone is expected to migrate outward due to the sinking of the underthrusting plate into the mantle. The formation of a back-arc or internal basin, i.e. of a basin internal to the surrounding belt of compression, (in this case the Tyrrhenian Sea) is then expected to take place if the motion of the overriding plate does not compensate for the retreat of the subduction zone. The sediment cover will be stripped from the underthrusting plate by the outward migrating arc of the overriding plate, and will accumulate to form an accretionary wedge. This accretionary body will grow outward in time, and will eventually become an orogenic belt, (in this case the present Apennines) when the migrating arc collides with the stable continental foreland on the subducting plate. An arc migration model satisfactorily accounts for the basic features of the Tyrrhenian-Apennine system and for its evolution from 17 Ma to the present, and appears to be analogous to the tectonic evolution of other back-arc settings both inside and outside the Mediterranean region. An interesting implication of the proposed accretionary origin of the Apennines is that the problematic “Argille Scagliose” (scaly clays) melange units might have been emplaced as overpressured mud diapirs, as observed in other accretionary prisms, and not by gravity slides from the internal zones.

Extension in the Tyrrhenian Sea and Shortening in the Apennines as Result of Arc Migration Driven by Sinking of the Lithosphere

To date, more than 7300 in situ stress orientations have been compiled as part of the World Stress Map project. Of these, over 4400 are considered reliable tectonic stress indicators, recording horizontal stress orientations to within <±25°. Remarkably good correlation is observed between stress orientations deduced from in situ stress measurements and geologic observations made in the upper 1–2 km, well bore breakouts extending to 4–5 km depth and earthquake focal mechanisms to depths of ∼20 km. Regionally uniform stress orientations and relative magnitudes permit definition of broad-scale regional stress patterns often extending 20–200 times the approximately 20–25 km thickness of the upper brittle lithosphere. The “first-order” midplate stress fields are believed to be largely the result of compressional forces applied at plate boundaries, primarily ridge push and continental collision. The orientation of the intraplate stress field is thus largely controlled by the geometry of the plate boundaries. There is no evidence of large lateral stress gradients (as evidenced by lateral variations in stress regime) which would be expected across large plates if simple resistive or driving basal drag tractions (parallel or antiparallel to absolute motion) controlled the intraplate stress field. Intraplate areas of active extension are generally associated with regions of high topography: western U.S. Cordillera, high Andes, Tibetan plateau, western Indian Ocean plateau. Buoyancy stresses related to crustal thickening and/or lithospheric thinning in these regions dominate the intraplate compressional stress field due to plate-driving forces. These buoyancy forces are just one of several categories of “second-order” stresses, or local perturbations, that can be identified once the first-order stress patterns are recognized. These second-order stress fields can often be associated with specific geologic or tectonic features, for example, lithospheric flexure, lateral strength contrasts, as well as the lateral density contrasts which give rise to buoyancy forces. These second-order stress patterns typically have wavelengths ranging from 5 to 10+ times the thickness of the brittle upper lithosphere. A two-dimensional analysis of the amount of rotation of regional horizontal stress orientations due to a superimposed local stress constrains the ratio of the magnitude of the horizontal regional stress differences to the local uniaxial stress. For a detectable rotation of 15°, the local horizontal uniaxial stress must be at least twice the magnitude of the regional horizontal stress differences. Examples of local rotations of SHmax orientations include a 75°–85° rotation on the northeastern Canadian continental shelf possibly related to margin-normal extension derived from sediment-loading flexural stresses, a 50°–60° rotation within the East African rift relative to western Africa due to extensional buoyancy forces caused by lithospheric thinning, and an approximately 90° rotation along the northern margin of the Paleozoic Amazonas rift in central Brazil. In this final example, this rotation is hypothesized as being due to deviatoric compression oriented normal to the rift axis resulting from local lithospheric support of a dense mass in the lower crust beneath the rift (“rift pillow”). Estimates of the magnitudes of first-order (plate boundary force-derived) regional stress differences computed from modeling the source of observed local stress rotations magnitudes can be compared with regional stress differences based on the frictional strength of the crust (i.e., “Byerlee's law”) assuming hydrostatic pore pressure. The examples given here are too few to provide a definitive evaluation of the direct applicability of Byerlee's law to the upper brittle part of the lithosphere, particularly in view of uncertainties such as pore pressure and relative magnitude of the intermediate principal stresses. Nonetheless, the observed rotations all indicate that the magnitude of the local horizontal uniaxial stresses must be 1–2.5+ times the magnitude of the regional first-order horizontal stress differences and suggest that careful evaluation of such local rotations may be a powerful technique for constraining the in situ magnitude stress differences in the upper, brittle part of the lithosphere.

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First‐ and second‐order patterns of stress in the lithosphere: The World Stress Map Project

splitting observations are interpreted in terms of the strain-induced lattice preferred orientation of mantle minerals, especially olivine. We consider three hypotheses concerning the origin of the continental anlsotropy: (1) strain associated with absolute plate motion, as in the oceanic upper mantle, (2) crustal stress, and (3) the past and present internal deformation of the subcontinental upper mantle by tectonic episodes. It is found that the last hypothesis is the most successful, namely that the most recent significant episode of internal deformation appears to be the best predictor of q. For stable continental regions, this is interpreted as "fossil" anisotropy, whereas for presently active regions, such as Alaska, the anisotropy reflects present-day tectonic activity. In the stable portion of North America there is a good correlation between delay time and lithospheric thickness; this is consistent with the anisotropy being localized in the subcontinental lithosphere and suggests that intrinsic anisotropy is approximately constant. The acceptance of this hypothesis has several implications for subcontinental mantle deformation. First, it argues for coherent deformation of the continental lithosphere (crust and mantle) during orogenies. This implies that the anisotropic portion of the lithosphere was present since the deformational episode and rules out the addition of undeformed material to this layer by subsequent "underplating" or conductive growth of the thermal boundary layer. One of the most important issues in the study of orogenies is the need to reconcile the formation of thickened lithosphere with the paradoxically high mantle temperatures often associated with orogenic episodes. Most efforts to date have focussed on modes of deformation whereby the cold lithospheric mantle is removed (by convective instability or delamination) and replaced by warm asthenosphere. These models, however, are incompatible with the evidence for preserved coherent lithospheric deformation; rather, the deformed mantle appears to have been heated in place. We suggest that the elevated mantle temperatures may be due to the strain heating accompanying the deformation.

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Shear wave splitting and subcontinental mantle deformation

Intraplate seismicity in continents is one of the important research topics of the International Lithosphere Program. It is expected that such research will improve our knowledge of deformation procedures and of the processes governing the release of seismic energy inside the continents, contribute to the mitigation of earthquake hazards, and provide a better understanding of intra-continental stress, and its spatial and temporal correlation with crustal heterogeneities. Deep continental drilling is a necessary and most important tool in this research, since in-situ physical, structural, compositional and rheological properties must be known at depth before an integrated model of intraplate seismicity with predictive capabilities can be established successfully. Drilling into seismic active areas in continents is therefore both a challenge and a necessity which requires the close cooperation of all geoscience disciplines.

Karl Fuchs

Papers

Rationale for a Deep Drill Hole Into the Seismically Active Area of the Hohenzollerngraben, Southwestern Germany