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

Construction of a crustal balanced cross-section across the Pyrenean chain shows a minimum shortening of 147 km, 112 km of which are related to stacking of basement thrust sheets in the southern Pyrenees. Metamorphic conditions of the basement rocks, as well as thrust geometry, indicate the maximum depth for the detachment level to be at 15 km. In the restored cross-section, the upper crust is 110 km longer than the lower layered crust. The lower crust was subducted together with the lithospheric mantle into the asthenospheric mantle and has not been imaged by geophysical data probably because an increase of density through eclogitic meta-morphism. The upper crust constitutes an orogenic lid mainly deformed by thrust structures. The balanced cross-section has been constrained by the ECORS deep reflection seismic profile as well as detailed surface data and available commercial seismic and oil well data. The restored cross-section provides a better picture of the middle Cretaceous combined strike-slip and extensional fault system as well as of the Hercynian crust. Hercynian geological features have been used as an additional tool for the restoration of the basement thrust sheets.

Evolution of a continental collision belt: ECORS-Pyrenees crustal balanced cross-section

In recent years, the origin of the Betic-Rif orocline has been the subject of considerable debate. Much of this debate has focused on mechanisms required to generate rapid late-orogenic extension with coeval shortening. Here we summarize the principal geological and geophysical observations and propose a model for the Miocene evolution of the Betic-Rif mountain belts, which is compatible with the evolution of the rest of the western Mediterranean. We regard palaeomagnetic data, which indicate that there have been large rotations about vertical axes, and earthquake data, which show that deep seismicity occurs beneath the Alboran Sea, to be the most significant data sets. Neither data set is satisfactorily accounted for by models which invoke convective removal or delamination of lithospheric mantle. Existing geological and geophysical observations are, however, entirely consistent with the existence of a subduction zone which rolled or peeled back until it collided with North Africa. We suggest that this ancient subducting slab consequently split into two fragments, one of which has continued to roll back, generating the Tyrrhenian Sea and forming the present-day Calabrian Arc. The other slab fragment rolled back to the west, generating the Alboran Sea and the Betic-Rif orocline during the early to middle Miocene.

https://spiral.imperial.ac.uk/bitstream/10044/1/21686/9/tect1148.pdf

Origin of the Betic-Rif mountain belt

Evidence is presented for azimuthal anisotropy in the subcontinental upper mantle from nine stations in North America and Europe. For the stations in the Canadian Shield, this is due to 2.5–2.7 Gyr-old 'fossil' anisotropy localized in the top 200km of the upper mantle and produced by the preferred orientation of mantle minerals. This mantle fabric, apparently caused by a series of large-scale deformational episodes at the end of the Archean, has moved coherently with the surface ever since.

Implications for continental structure and evolution from seismic anisotropy

We report on a marine seismic survey that images an active accretionary wedge west of Gibraltar. Ramp thrusts offset the seafloor and sole out to an east-dipping decollement, indicating ongoing westward-vergent tectonic shortening. New traveltime tomographic re- sults image a slab of oceanic lithosphere descending from the Atlantic domain of the Gulf of Cadiz, passing through intermediate-depth (60-120 km) seismicity beneath the west- ernmost Alboran Sea, and merging with a region of deep-focus earthquakes 600-660 km below Granada, Spain. Together, these new data provide compelling evidence for an active east-dipping subduction zone.

Evidence for active subduction beneath Gibraltar

Summary. In 1974 and 1975 a network of crustal seismic refraction profiles across the Betic Cordillera was established with shotpoints at sea off the Spanish coast. The longest profile with a length of 440 km runs parallel to the general strike through the centre of the Betic Cordillera. Two other profiles lie south of the main profile closer to the coast and perpendicular to the east-west strike, respectively. The interpretation of P waves indicates pronounced lateral variations of the structure and thckness of the crust and upper mantle. Two different crustal blocks seem to exist on either side of the north-east trending Palomares and Alhama de Murcia fault system in the eastern Betic Cordillera. East of this fault system the crust is only 23 km thick with a shallow high velocity layer of 6.9 km s-'. West of this line in the central and southern part of the Betic Cordillera the crust-mantle boundary lies at depths of 39 km and 24 km, respectively. In this area a velocity of 6 km s-l is reached at about 3 km depth. A layer of reduced P-wave velocity follows between 7 and 12 km depth with a velocity of 5.4 km s-'. At its lower boundary the velocity increases to 6.2 km s-l and reaches values between 6.6 and 6.7 km s-l at a depth of 18 km under the south coast and 24 km under the centre of the Betic Coddlera east of Granada. A thin lid of about 6 km thickness and a P, velocity of 8.1 km s-' lies on top of a rather thick layer with the reduced velocity of 7.8 km s-'. At a depth of 63 km the velocity increases again to 8.3 km s-'. Deep-reaching structural differences mark the east-west trending boundary between the internal and external zones of the central Betic Cordillera.

/pdf/crustal-structure-under-the-central-and-eastern-part-of-the-4txd0rvq9i.pdf

Crustal structure under the central and eastern part of the Betic Cordillera

Summary Quarry blasts recorded along three lines on the central Iberian Meseta are used in an attempt to interpret the crustal structure The results of the interpretation of the data, together with published surface wave and earthquake data, suggest a layered structure of the crust having the following features: the basement, in some areas covered by up to 4 km of sediments, has a P-velocity of 61 km s−1; a low-velocity layer, between 7 and 11 km depth, seems to exist on the basis of both P and S interpretation of seismic data; a thick middle crust of 12 km has a P-velocity of 64 km s−1 and overlies a lower crust with a mean P-velocity of 69 km s−1 and a possible slight negative gradient; the mean vp/vs ratio for the crust is about 175; the Moho is reached at about 31 km depth and consists of a transition zone at least 15 km thick The P-velocity of the upper mantle is close to 81 km s−1 and the S-velocity about 45 km s−1, which gives a vp/vs ratio of 18 for the uppermost mantle A tentative petrological interpretation of the velocities and composition of the layers is given

/pdf/crust-and-upper-mantle-structure-of-the-central-iberian-39peyjzlsv.pdf

Crust and upper mantle structure of the central Iberian Meseta (Spain)

Crustal structure under the western end of the Betic cordillera (Spain)

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Crustal structure under the central and eastern part of the Betic Cordillera

Crust and upper mantle structure of the central Iberian Meseta (Spain)

Crustal structure under the western end of the Betic cordillera (Spain)