It is widely assumed that the boundary layer above the core is the source of intraplate volcanoes such as Hawaii, Samoa, and Yellowstone, and that the sub-plate boundary layer at the top of the mantle is thin and entirely subsolidus. In fact, this upper layer is thicker and has higher expansivity, buoyancy, and insulating power than the lower one, and may have higher potential temperatures. The observed seismic structure of the low-velocity zone (LVZ) including attenuation, anisotropy, sharp boundaries, and a reduction of both compressional and shear moduli can be taken as strong evidence for the ubiquitous presence of melt in the upper mantle. If the LVZ contains as little as 1%–2% melt, then it is the most plausible and accessible source for midplate magmas; deeply rooted active upwellings are unnecessary. The upper boundary layer is also the most plausible source of ancient isotopic signatures of these magmas and their inclusions.

Seismic anisotropy is an efficient way to investigate the deformation field within the upper mantle. In the framework of rigid tectonic plates, we make use of recent tomographic models of azimuthal anisotropy to derive the best rotation pole of the Pacific plate in the uppermost 200 km of the mantle. It is found to be in good agreement with current plate motion (NUVEL1, HS3, and NNR). However, when dividing the Pacific plate into two subplates separated by what we refer to as the megagash, an east-west low-velocity and low-anisotropy band extending across the Pacific plate from Samoa-Tonga to the Easter–Juan Fernández Islands, the rotation pole of northern Pacific is still in agreement with current plate motion but not the rotation pole of the southern part of the Pacific, far away from the “classical” rotation pole of the Pacific plate. This result suggests a differential motion between the North and South Pacific and an ongoing reorganization of plates in the Pacific Ocean. The megagash might be a future plate boundary between the North and South Pacific plates, associated with the intense volcanism along this band.

Whether the volcanism of the Columbia River Plateau, eastern Snake River Plain, and Yellowstone (western U.S.) is related to a mantle plume or to plate tectonic processes is a long-standing controversy. There are many geological mismatches with the basic plume model as well as logical flaws, such as citing data postulated to require a deep-mantle origin in support of an “upper-mantle plume” model. USArray has recently yielded abundant new seismological results, but despite this, seismic analyses have still not resolved the disparity of opinion. This suggests that seismology may be unable to resolve the plume question for Yellowstone, and perhaps elsewhere. USArray data have inspired many new models that relate western U.S. volcanism to shallow mantle convection associated with subduction zone processes. Many of these models assume that the principal requirement for surface volcanism is melt in the mantle and that the lithosphere is essentially passive. In this paper we propose a pure plate model in which melt is commonplace in the mantle, and its inherent buoyancy is not what causes surface eruptions. Instead, it is extension of the lithosphere that permits melt to escape to the surface and eruptions to occur—the mere presence of underlying melt is not a sufficient condition. The time-progressive chain of rhyolitic calderas in the eastern Snake River Plain–Yellowstone zone that has formed since basin-range extension began at ca. 17 Ma results from laterally migrating lithospheric extension and thinning that has permitted basaltic magma to rise from the upper mantle and melt the lower crust. We propose that this migration formed part of the systematic eastward migration of the axis of most intense basin-range extension. The bimodal rhyolite-basalt volcanism followed migration of the locus of most rapid extension, not vice versa. This model does not depend on seismology to test it but instead on surface geological observations.

The first broad Russian experiment aimed at the study of the deep structure of Earth's crust and upper mantle by the common midpoint (CMP) method along the 1-EU geotraverse and cross-traverse 4B was realized in 1995–2008 in the territory of the East European Platform under the Russian Federal Program on Development of the State Geotraverse Network and Deep and Superdeep Boreholes. At the same time, the EGGI profile, geotraverses TATSEIS, ESRU 2003–2005, and DOBRE in Ukraine, as well as the system of profiles under the FIRE project in the adjacent territory of Finland were acquired. Integration of the existing geological maps and available geological (in the widest sense: structural, geochemical, geochronological, and so forth) data with results of geological interpretation of seismic images of the crust and upper mantle have led to a three-dimensional (3D) model of the deep crustal structure of the East European craton and a significant revision of previous ideas on the deep structure and Early Precambrian evolution of the region. In the geological interpretation of seismic data, we attached particular significance to the direct tracing of geological boundaries and fault zones recognized on the seismic-reflection pattern and the section of effective acoustic impedance toward the present-day surface and to their correlation with mapped geological and tectonic units. Comparison of the seismic image geometry with the geology of the eastern Fennoscandian Shield at the present-day erosion level shows that the reflection pattern matches the general trends of compositional layering, gneissic banding, and schistosity. The roughly homogeneous structural domains of the crust correspond to relatively large tectonic sheets, 3–5 km thick. Their inner structure commonly is not discernible in reflection patterns. The 3D model of deep structure in the Kola-Lapland region is based on correlation of tectonostratigraphic complexes depicted in the geological-tectonic map with structural subdivisions recognized as a result of interpretation of seismic crust images and their tracing to depth. In addition to the geological section along the 1-EU geotraverse, the model includes the section along the FIRE-4–4a profile that crosses the western part of the region studied in Finland. The 3D model shows the Paleoproterozoic tectonic structures (Lapland granulite-gneiss belt and its structural and evolutionary relationships with lower-crustal granulites, the structure and tectonic position of the Tana belt) and Archean tectonic structural units (the Central Kola granulite-gneiss belt, the Inari-Kola granite-greenstone domain, and the boundary zone between the Kola craton and the Belomorian orogen). The detailed 3D model of the crust and uppermost mantle in the Karelian-Belomorian region is also based on correlation between the exposed geological structure and geological interpretation of seismic images along the 1-EU geotraverse and cross-traverse 4B. The geological interpretation of the seismic crust image along the FIRE-1 profile serves as the additional basis for the Svecofennian accretionary orogen and its boundary with the Kola-Karelia continent. The model of the crust in the Karelian-Belomorian region contains the Paleoproterozoic tectonic structures (East Karelian imbricate thrust belt, Svecofennian accretionary orogen, and Onega Depression) and also Archean tectonic structures (Kuhmo-Segozero and Kovdozero microcontinents, and Chupa granulite-gneiss belt). The deep structure of the platform basement beneath the Moscow syneclise is an immediate extension of the Fennoscandian Shield. The basement structure in this area was controlled by Paleoproterozoic processes resulting in formation of the Lapland–Mid-Russia–South Baltia intracontinental orogen. The 3D model shows the marginal Totma and Aprelevka volcanic-sedimentary belts, and a synformal structure for the upper crust in the central domain of the orogen. The rock complexes of the Zubtsov-Diakonovo granulite-gneiss belt in the northwest and the Dmitrov-Galich belt in the southeast make up a distinctly outlined stage in the synform section. These complexes are underlain by gneiss-migmatite-amphibolite associations of the Bologoevo and Ivanovo-Sharya belts and are overlain by similar rocks of the Tver and Bukalovo belts. Lastly, the Kashin synformal granulite-gneiss belt is localized in the upper part of the section. The alternation of rocks of differing metamorphic grade clearly indicates the tectonic or tectonized stratigraphic character of the section in the Nelidovo synform. The crust sandwiched between the southward-plunging Totma and Aprelevka belts is characterized by rough layering. The reflections and boundaries of crustal sheets outlined in agreement with this pattern plunge southward beneath the Archean Sarmatia and Volgo-Uralia continental blocks. The deep crustal structure of the Voronezh Crystalline Massif is determined by a succession of geodynamic settings and Archean and Paleoproterozoic tectonic events that resulted in the formation of the Archean crust in the Kursk granite-greenstone domain and probably in the Khopior microcontinent, the Middle Paleoproterozoic East Voronezh orogen, and the Late Paleoproterozoic North Voronezh orogen. The 3D model applies particularly well to the Middle Paleoproterozoic East Voronezh orogen. The orogen is localized in the area of collision of the Kursk and Khopior microcontinents, which differ markedly in crustal structure and composition. The crocodile-type tectonic structure of the East Voronezh orogen is clear evidence for collisional compression. The countermotion of microcontinents resulted in the wedge-shaped structure of the Kursk microcontinent extending for 150 km, delamination of crust in the Khopior microcontinent, and counterdisplacement of tectonic sheets coherently thrust over and under the Kursk microcontinent. The tectonic structure of the central and western Volgo-Uralia continent to a depth of 15–20 km is characterized by sections of 3D models of effective density and magnetization. The second block of information on the Volgo-Uralia continent deep structure comes from results of seismic profiling along the TATSEIS, ESRU 2003–2005, and URSEIS geotraverses. The TATSEIS geotraverse crosses a significant part of the Volgo-Uralia continent from southeast to northwest. The seismic crust images along this geotraverse not only create the basis for interpretation of regional deep structure, but also robustly link the crustal models of the western and southeastern parts of the Volgo-Uralia continent. The data along the ESRU 2003–2005 geotraverse played an important role in ascertaining the deep structure of the Krasnoufimsk ovoid, which is overlapped by sedimentary fill of the Ural foredeep. Additional evidence was provided by the URSEIS geotraverse. The Archean crust, slightly modified in the Paleoproterozoic, which forms the East European Platform basement in the Volgo-Uralia continent, is made up of mafic granulites, khondalite, mafic-ultramafic intrusions, and granitoid plutons. The 3D model of the crust based on the TATSEIS geotraverse demonstrates the deep structure of the Vetluga synform in the Tokmov ovoid and of the interovoid domain. Ovoids play a crucial role in the Volgo-Uralia continent structure and occupy no less than 60% of the crust. In 3D representation, they are bowl-shaped blocks, round or oval in outline, and 300–600 km in diameter at the basement surface, and with a base at the level of the crust-mantle interface, i.e., at a depth of 60 km. The thickness of crust of the interovoid domain does not commonly exceed 50 km. Two types of elongated oval synforms are distinguished: the interovoid ovals (Verkhnevyatka, North Tatar, Almetevsk), up to 200–300 m long, with aspect ratio of 2:1–3:1, and the interovoid belts (Usovo, Vyatka, Kilmez, Elabuga-Bondyuga, Tuma-Penza, as well as Zhiguli-Pugachev homocline), 300–400 km in extent, with aspect ratio of 4:1–5:1. The bottom of the largest interovoid oval crossed by the TATSEIS profile reaches 25 km in depth. In crustal section, the structural elements of the interovoid domain are underlain or partly crosscut by acoustically transparent layers composed of the Bakaly-type granitoids. The lower crust of the interovoid domain is ~35 km in thickness and is composed of tectonic sheets plunging toward the northwestern end of the geotraverse and penetrating into the mantle.

Lithospheric foundering or delamination has been long recognized as an important process in the formation of the Andes, but the scale, timing, and surface uplift consequences remain controversial. We use recently completed ambient noise tomography and finite-frequency P-wave tomography results and other geologic and geophysical information to identify two ~200-km-diameter regions of piecemeal delamination in the Puna region between 21°S and 27°S. One location in the northern Puna Plateau is centered under the 11–1 Ma large-volume silicic Altiplano-Puna volcanic center, and the other in the southern Puna Plateau is centered approximately between the Arizaro Basin and 6–2 Ma Cerro Galan volcanic field. The foundering in the northern location has progressed to the point where the main thermal anomaly resides in the middle and upper crust, and the surface volcanic flare-up and mantle thermal anomalies are both in a waning stage. In the southern location, the main thermal anomaly is still in its waxing stage in the lower crust and upper mantle, and the foundering mantle material is imaged in the mantle wedge. The differing patterns of back-arc volcanism in the two foundering centers suggest different styles and timing of delamination, with the foundering process coming to completion earlier in the north than in the south. Based on plate-motion reconstructions, the NE-SW–aligned Juan Fernandez Ridge swept southward through this area starting about ca. 14 Ma in the north and ca. 10 Ma in the south. Although we do not think the passage of the Juan Fernandez Ridge initiated foundering, it played an important role in facilitating delamination by increasing interplate coupling, and weakening and perhaps hydrating the upper plate, and its passage allowed the delaminated material to sink into the expanding space of the mantle wedge. Another important factor in this evolution is the upper-plate lithospheric strength variations inherited from the different geologic basements underlying the northern and southern Puna regions. As the larger-scale delamination progressed, leaving behind thin lithosphere and a mantle wedge with a mixture of continental lithospheric fragments and hot asthenosphere, smaller secondary Rayleigh-Taylor instabilities occurred beneath the southern Puna Plateau, influencing basin development, and subsequent melting of this “drip” material was the source of the ensuing low-volume mafic volcanism.

The lack of field evidence for significant crustal stretching during the main phase of flood basalt eruption rules out a passive-mantle origin for the Columbia River Basalt Group. The short duration (1.1 m.y.) of the main-phase lavas, their large volume (~195,500 km3), and their high eruption rate (~0.178 km3/yr) in an area of minimal extension (<<1%) are better explained by an active-mantle upwelling origin. Such a scenario could be satisfied by the arrival of a lower-mantle plume, or by the ascent of upper mantle through a slab gap or around the truncated edge of the Farallon plate. These competing ideas have been based on the interpretation of seismic data beneath the Yellowstone region that appear to resolve either a sheet of mantle upwelling extending downward into the mantle transition zone, or an irregular corridor of upwelling extending through the transition zone and into the lower mantle to ~900–1000 km depth. A lower-mantle origin is supported by the seismically resolved upward deflection in the ~660 km discontinuity at the base of the transition zone, which in turn is consistent with a lower-mantle chemical signature reflected in high 3He/4He ratios found in derivative basalts from both the Snake River Plain and the Columbia River flood basalt province. Mid-Miocene arrival of the Yellowstone plume was centered beneath the Oregon-Nevada border region, where there are direct spatial and temporal connections among the initiation of flood basalt volcanism at ca. 16.7 Ma, the initiation of rhyolitic volcanism at the western end of the Snake River Plain hotspot track at ca. 16.5 Ma, and the initiation of a major period of uplift and crustal extension in the northern Basin and Range between ca. 17 and 16 Ma. Rapid uplift and the propagation of volcanism away from this region are consistent with models of plume emplacement. The inability of seismic studies to image a mantle-plume fabric of radiating flow is due to its destruction by corner flow, which is evident in the contemporary east-west flow fabric derived from shear-wave splitting measurements.

The Kings-Kaweah ophiolite belt of the southwestern Sierra Nevada Foothills was generated in two pulses of mid-oceanic-ridge basalt (MORB) magmatism. The first was in the Early Ordovician, which resulted in the generation of a complete abyssal crust and upper mantle section. The crustal section was rendered from convecting mantle whose Nd, Sr, and Pb isotopic systematics lie at the extreme end of the sub-Pacific mantle regime in terms of time integrated depletions of large ion lithophile (LIL) elements. Semi-intact fragments of this Early Ordovician oceanic lithosphere sequence constitute the Kings River ophiolite. Following ~190 m.y. of residence in the Panthalassa abyssal realm, a second pulse of MORB magmatism invaded the Early Ordovician lithosphere sequence in conjunction with intensive ductile shearing and the development of ocean floor mélange. This Permo-Carboniferous magmatic and deformational regime produced many of the essential features observed along spreading ridge–large-offset transform fracture zones of the modern ocean basins. During this regime, Early Ordovician upper mantle–lower crustal rocks were deformed in the ductile regime along what appears to have been an oceanic metamorphic core complex, as well as along steeply dipping strike-slip ductile shear zones that broke the ophiolite into semi-intact slabs. Progressive deformation led to the development of serpentinite-matrix ophiolitic mélange within the abyssal realm. This (Kaweah) serpentinite mélange constitutes the majority of the ophiolite belt and encases fragments of both disrupted Early Ordovician oceanic lithosphere and crustal igneous-metamorphic assemblages that were deformed and disrupted as they formed by diffuse spreading along the fracture zone. An ~190 m.y. hiatus in abyssal magmatism cannot be readily accommodated in the current configuration of Earth's ocean basins, but it was possible during the mid- to late Paleozoic Panthalassa regime, when the proto-Pacific basin occupied over half of the Earth's surface. The transform history of the ophiolite belt can be directly linked to the late Paleozoic transform truncation of the SW Cordilleran passive margin. Following juxtaposition of the transform ophiolite belt with the truncated margin a change in relative plate motions led to the inception of east-dipping subduction, and the en masse accretion of the ophiolite belt to the hanging wall of the newly established subduction zone. Structural relations and isotopic data on superimposed igneous suites show that the ophiolite belt was not obducted onto the SW Cordilleran continental margin. The accreted ophiolite belt formed the proto-forearc of the newly established active margin. The ophiolite belt never saw high-pressure/temperature (P/T) metamorphic conditions. Rare small blocks of high-pressure metamorphic rocks were entrained from the young subduction zone by serpentinite diapirs and emplaced upward into the ophiolitic mélange within a proto-forearc environment. An Sm/Nd garnet-matrix age on a high-pressure garnet amphibolite block suggests subduction initiation at ca. 255 Ma. This timing corresponds well with the initiation of arc magmatism along the eastern Sierra Nevada region. In Late Triassic to Early Jurassic time proximal submarine mafic eruptions spread across and mingled with hemipelagic and distal volcaniclastic strata that were accumulating above the accreted ophiolite belt. These lavas carry boninitic to arc tholeiitic and primitive calc-alkaline geochemical signatures. By Middle Jurassic time siliciclastic turbidites derived from early Paleozoic passive margin strata and early Mesozoic arc rocks spread across the primitive forearc. In late Middle to Late Jurassic time tabular plutons and dike swarms of calc-alkaline character invaded the ophiolite belt in a transtensional setting. Deformation fabrics that developed in these intrusives, as well as cleavage that developed in the cover strata for the ophiolite belt, imparted components of superimposed finite strain on the ophiolitic mélange structure but did not contribute significantly to mélange mixing. By ca. 125 Ma, copious gabbroic to tonalitic plutonism of the western zone of the Cretaceous Sierra Nevada batholith intruded the ophiolite belt and imparted regional contact metamorphism. Such metamorphism variably disturbed U/Pb systematics in rare felsic intrusives of the ophiolite belt but did not significantly disturb whole rock Sm/Nd systematics. Age constraints gained from the Sm/Nd and U/Pb data in conjunction with Nd, Sr, and Pb isotopic and trace element data clearly define the polygenetic abyssal magmatic history of the ophiolite belt. The variation of Nd and Sr radiogenic isotopes over time from the Paleozoic abyssal assemblages, through early Mesozoic supra-subduction zone volcanism to Early Cretaceous batholithic magmatism, record the geochemical maturation of the underlying mantle wedge without the involvement of SW Cordilleran continental basement.

To further advance our understanding of the way in which a portion of the African superswell in eastern Africa formed, and also to draw attention to the importance of eastern Africa for the plume versus plate debate about mantle dynamics, upper-mantle structure beneath eastern Africa is reviewed by synthesizing published results from three types of analyses applied to broadband seismic data recorded in Tanzania, Kenya, and Ethiopia. (1) Joint inversions of receiver functions and surface wave dispersion measurements show that the lithospheric mantle of the Ethiopian Plateau has been significantly perturbed, much more so than the lithospheric mantle of the East African Plateau. (2) Body wave tomography reveals a broad (≥300 km wide) and deep (≥400 km) low-velocity anomaly beneath the Ethiopian Plateau and the eastern branch of the rift system in Kenya and Tanzania. (3) Receiver function stacks showing Ps conversions from the 410 km discontinuity beneath the eastern branch in Kenya and Tanzania reveal that this discontinuity is depressed by 20–40 km in the same location as the low-velocity anomaly. The coincidence of the depressed 410 km discontinuity and the low-velocity anomaly indicates that the low-velocity anomaly is caused primarily by temperatures several hundred degrees higher than ambient mantle temperatures. These findings cannot be explained easily by models invoking a plume head and tail, unless there are a sufficient number of plume tails presently under eastern Africa side-by-side to create a broad and deep thermal structure. These findings also cannot be easily explained by the plate model. In contrast, the breadth and depth of the upper-mantle thermal structure can be explained by the African superplume, which in some tomographic models extends into the upper mantle beneath eastern Africa. Consequently, a superplume origin for the anomalous topography of the African superswell in eastern Africa, in addition to the Cenozoic rifting and volcanism found there, is favored.

The African continent preserves a long geological record that covers almost 75% of Earth's history. The Pan-African orogeny (c. 600–500 Ma) brought together old continental kernels (or cratons such as West African, Congo, Kalahari and Tanzania) forming Gondwana and subsequently the supercontinent Pangea by the late Palaeozoic (Fig. 1).The break-up of Pangea since the Jurassic and Cretaceous, primarily through the opening of the Central Atlantic (e.g. Torsvik et al. 2008; Labails et al. 2010), Indian (e.g. Gaina et al. 2007; Müller et al. 2008; Cande et al. 2010) and South Atlantic (e.g. Torsvik et al. 2009) oceans and the complicated subduction history to the north gradually shaped the African continent and its surrounding oceanic basins. Many first-order questions of African geology are still unanswered. How many accretion phases do the Proterozoic belts represent? What triggers extension and formation of the East African Rift on a continent that is largely surrounded by spreading centres and, therefore, expected to be mainly in compression? What is the role of shallow mantle and edge-driven convection (King & Ritsema 2000)? What are the sources of the volcanic centres of Northern Africa (e.g. Tibesti, Dafur and Afar) and can they be traced to the lower mantle? Is the elevation of Eastern and Southern Africa caused by mantle processes? What is the formation mechanism of intracratonic sedimentary basins, such as the Taoudeni Basin on the West African Craton and the Congo

Explaining the cause and support of Africa's varied topography remains a fundamental question for our understanding of the long-term evolution of the continent. As geodynamical modelling becomes more frequently used to investigate this problem, it is important to understand the seismological results that can be incorporated into these models. Crustal thickness estimates are crucial for calculating components of topography that are isostatically compensated. Variations in seismic velocity help constrain variations in subsurface temperature and density and thus buoyancy; measurements of anisotropy can also be used to determine the contribution of the mantle flow field to dynamic topography. In this light, we review the results of passive seismic studies across Africa. At the continental scale there are significant differences in crustal models, meaning large uncertainties in corrections for isostatic topography. In east Africa, multiple seismic experiments have provided firm constraints on crustal and mantle structure. Tomographic images illuminate a broad (c. 500 km wide) low-velocity region in the upper mantle, with possible connection to the African Superplume in the lower mantle. These observations, alongside the variations in radial anisotropy, strongly suggest that the mantle flow field contributes significantly to the uplift of the region. Beneath southern Africa, low velocities are observed near the base of the continental lithosphere; the depth to transition zone discontinuities however suggests that they are not linked to the superplume beneath. It is thus less clear what role the sublithospheric mantle plays in supporting the region's high topography. Many of Africa's secondary topographic features (e.g. Atlas, Hoggar, Bie Dome) are underlain by slow velocities at depths of 100–150 km and are adjacent to rapid changes in lithospheric thickness. Whether these variations in lithospheric structure promote small-scale convection or simply guide the larger-scale mantle flow field remains ambiguous.

We combine horizontal Global Positioning System (GPS) velocities from a new compilation of published and new GPS velocities, results from an interferometric synthetic aperture radar (InSAR) study, and paleoseismic data to evaluate the postseismic response of historic earthquakes in the Central Nevada seismic belt. We assume that GPS velocity has contributions from time-invariant (i.e., steady permanent crustal deformation) and transient (i.e., time varying and associated with the seismic cycle) processes that are attributable to postseismic viscoelastic relaxation of the crust and upper mantle. In order to infer the viscosity structure of Basin and Range lower crust, ηLC, and upper mantle, ηUM, we apply three objective criteria to identify rheological models that fit both geodetic and geologic data. The model must (1) improve the apparent mismatch between geodetically and geologically inferred slip rates, (2) explain the InSAR-inferred vertical uplift rate, and (3) not imply time-invariant contractions anywhere in the extending province. It is not required for the postseismic deformation field to resemble the time-invariant velocity field in pattern, rate, or style. We find that the InSAR and horizontal GPS velocities form complementary constraints on the viscoelastic structure, excluding different parts of the model space. The best-fitting model has a lower crust that is stronger than the uppermost mantle, with ηLC = 1020.5 Pa·s and ηUM = 1019 Pa·s, a finding consistent with the majority of similar studies in the Basin and Range. The best-fitting viscosity model implies that the majority of Central Nevada seismic belt deformation is attributable to postseismic relaxation, and hence that western Basin and Range time-invariant deformation north of 39°N latitude is more tightly focused into the northern Walker Lane than would be inferred from uncorrected GPS velocities. However, significant deformation remains after correction for postseismic effects, consistent with Central Nevada seismic belt faults slipping at rates intermediate between the Walker Lane belt and the central Basin and Range.

An analysis of seismicity in the northern Mojave block, southern Walker Lane belt, and southern Sierra Nevada provides a detailed snapshot of the kinematics of active deformation within a young and probably evolving intracontinental plate boundary in eastern California. Earthquakes in this region were relocated using joint hypocentral inversion, double-difference, and waveform cross-correlation techniques. Groups of focal mechanisms were inverted for the components of a reduced deformation rate tensor, and the inversion results were synthesized in maps of the regional seismogenic deformation field. In general, seismogenic deformation in the Eastern California shear zone and Walker Lane belt is characterized by distributed simple shear and primarily accommodates northwest translation of the Sierra Nevada–Central Valley (Sierran) microplate relative to stable North America. Crustal thinning in the southern Walker Lane belt is subordinate to northwest-directed dextral shear and typically is associated with releasing geometries among dextral faults. There is no significant variation in strain geometry in the transition from the Eastern California shear zone to the Walker Lane belt across the Garlock fault; we interpret this to indicate that this structure is being sheared and rotated clockwise by distributed northwest dextral shear. In contrast, seismogenic deformation in the southeastern Sierra Nevada near latitude 36°N is characterized by horizontal elongation and locally by flattening, i.e., extension in two perpendicular horizontal directions. The transition from wrench tectonics in the southern Walker Lane to flattening in the High Sierra occurs across a 25- to 35-km-wide transtensional zone west of the Sierran range front. The elongation and flattening occur directly east of the “Isabella anomaly,” a high-velocity anomaly in the upper mantle interpreted to be lower Sierran lithosphere that is foundering or convectively descending into the asthenosphere. The seismogenic thinning of the southern Sierra may be related to thickening above the descending lithosphere to the west and/or may be driven by tensile buoyancy forces. The seismotectonics of this region may provide insights into the process by which the Walker Lane belt has progressively expanded westward into the Sierran microplate during late Cenozoic time.

The nature of petrologic and structural properties and processes that characterize the middle and lower continental crust is a long-standing problem in the earth sciences. During the past several decades significant progress has been made on this fundamental problem by synthesizing deep-crustal seismic-reflection imaging, laboratory-based seismic-velocity determinations, xenolith studies, and detailed geologic studies of exposed crustal cross sections. Geological, geochemical, and geophysical studies of crustal sections provide a crustal-scale context for a variety of important problems in the earth sciences. Crustal sections are widely used to evaluate crustal composition and petrogenesis, including lateral and vertical variations in rock types. Evidence from deep levels of crustal sections suggests seismic shear-wave anisotropy and seismic lamination result from widespread subhorizontal contacts, shear zones, and transposition fabrics, and in some sections from metamorphosed m- to km-thick, intraplated and/or underplated mafic magmatic sheets and plutons. Crustal sections also facilitate the evaluation of crustal rheology in natural settings from regional to outcrop scale. Magmatism, metamorphism, partial melting, and relatively small lithological differences control rheology, localize strain, and lead to markedly heterogeneous deformation over a wide range of crustal levels. Finally, crustal sections provide unique views of the architecture and deformation patterns of fault zones in the deep crust. As a guide to the growth and evolution of continental crust in the past 0.5 Ga, we summarize the salient features of some examples of crustal cross sections from Phanerozoic orogens. These crustal sections represent different tectonic settings, although the variation in magmatic arcs from intra-oceanic to continental-margin settings is a major theme in our synthesis. Another theme is the importance of attenuated crustal sections in reconstructing the hinterland of orogens that have experienced large-magnitude crustal extension after an earlier history of crustal contraction. The Phanerozoic crustal cross sections summarized in this chapter developed during a polyphase deformational and magmatic history that spanned 10–100s of Ma and resulted in overprinting of different events. Consequently, we conclude that there is no “typical” Phanerozoic continental crustal section, and the overall crustal composition varies markedly between sections. The thickness of lower crust that existed below an exposed crustal section is difficult to quantify. Only a few sections are in contact (typically faulted) with mantle rocks, and although xenoliths can provide important information about the unexposed parts of the deep crust and upper mantle, they are absent for most sections. The exhumation of relatively intact crustal cross sections and lower-crustal rocks probably requires an unusual sequence of tectonic events, and almost all of the sections evaluated in this chapter were exhumed by multiple mechanisms. Major exhumation is most commonly attributed to normal faults and extensional shear zones.

One of the critical questions of continental dynamics is whether the preliminary assumption of rigid plate rotation is applicable to a continental lithosphere that possesses a long history of geological evolution. Different theoretical models give diverse answers that are difficult to test because of the lack of coherent and reliable kinematical constraints. In this paper, we study kinematic patterns of present-day tectonic deformation (or crustal movement) of continental China on the basis of 1350 global positioning system (GPS) measurements in combination with active faults and seismic activity studies. The present-day tectonic deformation of continental China is characterized by a combination of rigid-block movement and continuous deformation. For example, the Tarim Basin, the Ordos, and South China behave as coherent blocks similar to oceanic rigid blocks without internal deformation, whereas the Tibetan Plateau and Tianshan seem to deform continuously with significant internal deformation. Mechanical and rheological properties of the lithosphere dictate the style of regional deformation. Regions of high rigidity behave with rigid block-like movements, and those of low rigidity are dominated by continuous deformation. Rheological flow in the lower crust and upper mantle plays an important role in controlling deformation of the upper crust. Present-day tectonic deformation of continental China can be described in terms of a combined model of rigid-block movement and continuous deformation.