Abstract The Fosdick migmatite–granite complex of West Antarctica preserves evidence of two crustal differentiation events along a segment of the former active margin of Gondwana, one in the Devonian–Carboniferous and another in the Cretaceous. The Hf–O isotope composition of zircons from Devonian–Carboniferous granites is explained by mixing of material from two crustal sources represented by the high-grade metamorphosed equivalents of a Lower Palaeozoic turbidite sequence and a Devonian calc-alkaline plutonic suite, consistent with an interpretation that the Devonian–Carboniferous granites record crustal reworking without input from a more juvenile source. The Hf–O isotope composition of zircons from Cretaceous granites reflects those same two sources, together with a contribution from a more juvenile source that is most evident in the detachment-hosted, youngest granites. The relatively non-radiogenic ɛHf isotope characteristics of zircons from the Fosdick complex granites are similar those from the Permo-Triassic granites from the Antarctic Peninsula. However, the Fosdick complex granites contrast with coeval granites in other localities along and across the former active margin of Gondwana, including the Tasmanides of Australia and the Western Province of New Zealand, where the wider range of more radiogenic ɛHf values of zircon suggests that crustal growth through the addition of juvenile material plays a larger role in granite genesis. These new results highlight prominent arc-parallel and arc-normal variations in the mechanisms and timing of crustal reworking v. crustal growth along the former active margin of Gondwana. Supplementary material: Figs S1 and S2 are available at http://www.geolsoc.org.uk/SUP18625

Abstract Intracontinental deformation occurred in West Antarctica during the final stages of plate convergence along the Cretaceous Gondwana margin. In western Marie Byrd Land, 115 Ma to 95 Ma A-type granitoids and mafic dykes record a change in plate kinematics. The magmatism typically is viewed as a record of extension leading to orthogonal break-up between New Zealand and Marie Byrd Land by c . 67 Ma. This paper presents new kinematic and 40 Ar/ 39 Ar age data for a mafic dyke array in the Ford Ranges, a region >1000 km 2 dominated by plutonic and metamorphic bedrock. The mean dyke trend of N16W corresponds to a maximum finite strain axis orientated N74E, highly oblique to the N58E-trending margin and to on-land crustal structures defined from airborne geophysics. 40 Ar/ 39 Ar emplacement ages for most dykes fall between 114 Ma and 97 Ma, coeval with emplacement of a gneiss dome at 101-96 Ma and with development of mylonitic shear zones at 100-95 Ma in coastal western Marie Byrd Land. The oblique orientation of maximum finite strain with respect to large faults, geophysical lineaments and the rifted margin of western Marie Byrd Land is consistent with transcurrent tectonics along this segment of the Gondwana margin at c . 100 Ma.

Various mechanisms have been proposed for the dynamic cause and kinematic development of gneiss domes. They include (1) diapiric flow induced by density inversion, (2) buckling under horizontal constriction (i.e., extension perpendicular to compression), (3) coeval orthogonal contraction or superposition of multiple phases of folding in different orientations, (4) instability induced by vertical variation of viscosity, (5) arching of corrugated detachment faults by extension-induced isostatic rebound, and (6) formation of doubly plunging antiforms induced by thrust-duplex development. Despite proliferation of models for gneiss-dome formation, a diagnostic link between the observed geological setting of a gneiss dome and the associated deformational processes remains poorly understood. This is because gneiss domes reflect finite-strain patterns that can be reached through different strain paths or superposition of multiple mechanisms. To better differentiate the competing mechanisms and to assist the clarity of future discussion, a classification scheme of individual gneiss domes and gneiss-dome systems is proposed. The scheme expands the traditional definition of mantled gneiss domes, which emphasizes the spatial association with synkinematic migmatite and a supracrustal cover, by including those associated with faults. To illustrate possible kinematic interactions between faulting and gneiss-dome development, major geologic properties of two end-member fault-related gneiss domes are discussed: one produced by the development of North American Cordilleran-style extensional detachment faults and the other by passive-roof thrusts in crustal-scale fault-bend folds. Distinguishing the two has become a critical issue in the Himalayan orogen and the western U.S. Cordilleran where emplacement and exhumation of gneiss domes have been variably interpreted to be detachment or thrust related. A systematic examination of gneiss domes related to contractional versus extensional faults indicates that a detachment-related gneiss dome is characterized by the presence of a breakaway system in the footwall, rapid footwall denudation by normal faulting, and coeval development of supradetachment basins in the hanging wall. In contrast, a gneiss dome related to passive-roof faulting is commonly associated with rapid denudation of both hanging-wall and footwall rocks by erosion and the lack of coeval supradetachment basins. The most important aspect of a gneiss-dome system is the spacing between individual domes. The evenly spaced gneiss-dome systems tend to be associated with instabilities induced by density inversion, vertical viscosity variation, or horizontal contraction in a laterally homogenous medium. In contrast, unevenly spaced gneiss-dome systems may be associated with fault development, superposition of multiple folding events, or laterally inhomogeneous properties of rocks comprising the gneiss-dome systems. In nature, gneiss domes are often produced by superposition of several dome-forming mechanisms. This has made determination of the dynamic cause of individual domes and dome systems exceedingly challenging.

Gneiss domes are ubiquitous structures in all exhumed orogens, and their formation represents a first order thermal-tectonic process that has operated from the Archean to the present. The vertical flow of crust to create domal structures is a significant factor in the redistribution of heat and material in orogens and therefore in the evolution of continents. Worldwide, gneiss domes display many similarities in geometry (aspect ratio), petrology, and structure, and these similarities transcend differences in tectonic setting. Gneiss domes are cored by high-grade metamorphic rocks (including migmatite) ± granitoids, and the core rocks commonly record a component of isothermal decompression, in contrast to mantling schists, and may exhibit a late, low-pressure–high-temperature metamorphic assemblage. Rapid cooling typically follows isothermal decompression, as hot rocks are rapidly emplaced at higher structural levels. Most gneiss domes are elongate parallel to the strike of the orogen. Domes with long dimension ≤90 km have a ratio of long to short axes of ∼2:1–3:1. The elliptical shape of gneiss domes worldwide suggests that their morphology, and therefore genesis, is controlled by crustal flow dynamics, including the magnitude of vertical versus lateral crustal flow. The conditions and mechanisms involved in dome formation inform the relative rates of vertical and lateral crustal flow during orogeny.

Gneiss domes are typically bounded by shear zones that accommodated differential exhumation relative to their surrounding host rocks. Consideration of possible structural geometries shows that two arrangements are particularly effective at exhuming deep crustal rocks. One consists of sub-parallel dipping shear zones, where an upper normal shear zone overlies a lower reverse shear zone. The other is a bivergent wedge, bounded by conjugate reverse shear zones, where vigorous erosion is required for substantial exhumation to occur. For the bivergent wedge to remain stable, material lost through erosion must be balanced by a combination of accretion via downward migration of the bordering shear zones and advection of material into the wedge. Symmetric wedges can only exhume deep rocks if either buckling or diapiric flow is significant. Asymmetric wedges are bounded by a dominant and a subordinate antithetic shear zone and can always exhume deep rocks. The petrologic signature of the asymmetric bivergent wedge is a thick zone of penetratively sheared rocks on the dominant side. If the antithetic shear zone becomes inactive, it may be eroded entirely, and the resulting structure may resemble a simple thrust nappe. The western and eastern Himalayan syntaxes are experiencing rapid exhumation. The western syntaxis contains an asymmetric bivergent wedge that formed in response to continued shortening after development of a crustal-scale antiform. In the eastern syntaxis, a bivergent wedge is found in one limb of a major antiform. This may represent a reactivated mid-crustal duplex, implying a different structural evolution than for the western syntaxis.

Domes and basins are evidence for vertical movements in both compression and extension tectonic environments. They are thus evidence for interplay between gravity and tectonic forces in structuring the continental crust. We employ analytical and numerical techniques to investigate the respective roles of gravity and compression during the growth of crustal-scale buckle anticlines and diapirs submitted to instantaneous erosion. The analytical perturbation method, which explores the onset of both types of instability, yields a “phase-diagram” discriminating eight folding-diapirism modes, five of which are geologically relevant. Numerical simulations show that the phase diagram is applicable to evolved, finite amplitude stages. Calculated strain fields in both diapirs and folds show normal sense of shear at the interface if the upper layer is thick compared to the lower layer. We conclude that the present-day structural techniques applied for distinguishing diapiric domes and folds are ambiguous if detachment folding and intense erosion take place during deformation, and that domes displaying extensional structures do not necessarily reflect extension.

We use a thermomechanical modeling approach to study the development of extensional gneiss domes in a thickened and thermally relaxed lithosphere. Our models consider a compositional and thermally dependent rheological lithosphere layering, with a 60-km-thick crust and Moho temperatures in the range 840–1040 °C. No discontinuity or detachment fault is assumed to preexist within the upper crust. However, to initiate localized deformation, a density anomaly is placed at the base of middle crust. Extension is applied to one model boundary at constant rates of 2.0 and 0.66 cm/yr. Models illustrate the progressive development of domes and associated strain patterns at crustal scale. Extension first localizes in the upper crust as a nearly symmetrical graben, allowing the underlying middle and lower ductile crust to rise up, initiating a dome. Dome amplification is further accommodated by convergent channel flow in the lower crust. Strain localization displays a complex pattern of shear zones at the crustal scale, first nearly symmetrical, and progressively becoming asymmetrical, giving, in particular, an upward convex detachment on one side of the dome. During extension, Moho geometry and depth vary as a function of boundary displacement rate. At the lower boundary displacement rate used in the calculations, the Moho remains rather flat and rises up at a constant rate. Results are discussed in the light of field examples and compared to previous models.

A quantitative model of gneiss dome formation must account for the spontaneous development of large structural relief, relative to surface relief, at the base of the brittle layer for appropriate physical parameterization of the crustal rheology and the modification of topography by erosion. An earlier model has been augmented to include (i) crustal necking, as well as contributions to dome initiation from (ii) density instability and from (iii) an erosional law in which sinusoidal components in surface topography are amplified at wavelengths L > L * and decay for L < L *. A further modification replaces a ductile halfspace with a downward softening viscous layer resting on rigid mantle at a weak planar detachment. Necking alone predicts crustal instability for only a few candidate creep laws and maximum rock strength near the upper surface. Density instability and erosional amplification make independent, additional contributions to the strength of instability of the same magnitude as that from necking, extending the domain of dome initiation to much larger ranges in the dimensionless parameters that govern model behavior. Our results indicate that crustal extension tends to strongly localize for a broad range of rock types dominating crustal rheology, leading to crustal necks centered on large amplitude antiformal structures. The tendency for the necks to form spontaneously and amplify rapidly is strongly enhanced by a destabilizing density contrast at depth and erosional evacuation of material from the necks. The analysis shows that crustal necks with a length scale of 50–100 km, 4–5 times the thickness of the brittle crust, tend to dominate the initial structures. These are likely to develop into domal structures, river anticlines, and, more generally, zones of localized deformation and rock uplift, especially where erosion is rapid.

Modeling of in situ rock properties based on a Gibbs free energy minimization approach shows that regional metamorphism of granulite facies may critically enhance the decrease of crustal density with depth. This leads to a gravitational instability of hot continental crust, resulting in regional doming and diapirism. Two types of crustal models have been studied: (1) lithologically homogeneous crust and (2) heterogeneous , multilayered crust. Gravitational instability of relatively homogeneous continental crust sections is related to a vertical density contrast developed during prograde changes in mineral assemblages and the thermal expansion of minerals with increasing temperature. Gravitational instability of lithologically heterogeneous crust is related to an initial density contrast of dissimilar intercalated layers enhanced by high-temperature phase transformations. In addition, the thermal regime of heterogeneous crust strongly depends on the pattern of vertical interlayering: A strong positive correlation between temperature and the estimated degree of lithological gravitational instability is indicated. An interrelated combination of two-dimensional, numerical thermomechanical experiments and modeling of in situ physical properties of rocks is used to study the processes of gravitational redistribution within a doubly stacked, heterogeneously layered continental crust. It is shown that exponential lowering of viscosity with increasing temperature, in conjunction with prograde changes in metamorphic mineral assemblages during thermal relaxation after collisional thickening of the crust, provide positive feedback mechanisms leading to regional doming and diapirism that contribute to the exhumation of high-grade metamorphic rocks.

Diapir and dome structures on a scale of meters to kilometers are widespread in Earth's continental crust and represent an important tectonic element of cratons, orogenic belts, and sedimentary basins. These structures advect heat from lower to higher crustal levels, often producing pronounced prograde contact metamorphic aureoles. This standard thermal situation is violated by the up to 8 km in diameter migmatitic domes and diapirs of metasedimentary rocks that penetrate the world's largest layered intrusion, the Bushveld Complex. These domes and diapirs rose with an average rate of about 8 mm/yr and were characterized by an unusual inverted thermal structure, with the cores of the structures 200–300 °C colder than the rims. Numerical modeling supports the interpretation that the process was triggered by the emplacement of an 8-km-thick, hot, dense mafic magma over a cold, less dense sedimentary succession, resulting in a dramatic lowering of the viscosity of the sediments during contact metamorphism and partial melting. Dome and diapir nucleation is interpreted to have been defined by the formation of initial anticline-shaped disturbances related to fingered lateral injection of the Lower Zone of the Bushveld intrusion between the felsic roof and sedimentary floor sequence. The partially molten, mobile, but relatively cold domes and diapirs promoted cooling of the giant magma chamber, rapidly bringing cooler material into higher crustal levels, and freezing the surrounding magmas. We argue that our work has a more general significance as similar thermal structures should be a widespread feature associated with partially molten mantle diapirs (“cold plumes”) generated in the proximity of subducting slabs. These structures are likely responsible for rapid upward melt transport above subduction zones and for the associated volcanic activity. The exposed structures observed in the Bushveld Complex provide a unique opportunity to study the “cold” diapir/plume phenomenon, thus leading to a broader recognition and understanding of this geological process.

Many high-grade metamorphic terrains record isothermal decompression, implying rapid exhumation or heat input during decompression. These terrains commonly contain gneiss domes that are spatially and temporally associated with low-angle normal faults such as those bounding metamorphic core complexes. To understand the thermomechanical relationship of gneiss domes and core complexes, we use two-dimensional numerical modeling to evaluate exhumation and cooling rates resulting from diapiric ascent of partially-molten crust, a proposed mechanism for gneiss dome formation, versus exhumation of orogenic crust by low-angle normal faulting, the primary unroofing mechanism in metamorphic core complexes. Pressure-temperature-time paths calculated for vertical ascent rates of 2–20 km/m.y. show that isothermal decompression is possible for rocks within a diapir. In contrast, paths calculated for rocks in the footwall of low-angle normal faults show that cooling occurs as rocks are brought closer to Earth's surface, and the rate at which these rocks cool is controlled by fault dip, displacement rate, and amount of displacement. The amount of heat loss per unit time during decompression increases with fault dip. For exhumation of rocks in the footwall of a very low-angle fault (∼10°), decompression paths occur with little cooling (quasi-isothermal), but the shallow dip of the fault does not allow significant decompression. Low-angle normal faulting alone cannot result in isothermal decompression: The presence of gneiss domes in core complexes requires an additional exhumation process, such as diapiric ascent or a more structurally and thermally complex evolution of the detachment system. In the late stages of exhumation, once the rocks have risen to depths of <15 km and experience rapid cooling, detachment faulting may be the primary mechanism of the final unroofing and juxtaposition of formerly deep rocks and upper crustal rocks.

In southeast Karakorum (northwest Himalaya, Pakistan), kilometric size migmatitic domes were exhumed in a context of north-south shortening during Neogene times. The domes are characterized by a conical shape, and ductile deformation criteria indicate both radial expansion and extrusion of the migmatitic core relative to the surrounding gneisses. Most of the domes are aligned along the dextral, strike-slip Shigar fault that is parallel to the N130°E Karakorum fault. Along the Shigar fault, exhumation of the domes is mainly vertical with a slight dextral component. We propose that the high temperature exhumation of the domes is due to diapiric ascent of the molten mid-crust helped by the compressive regime. The localization of the initial diapir was controlled by crustal-scale vertical structures parallel to the Karakorum fault. The later stage of exhumation in mid to low temperature conditions was related to the uplift and erosion of the whole southeastern Karakorum by crustal-scale east-west folding. In south Tibet, the westward prolongation of south Karakorum, Neogene crustal melting is also supported by geophysical data and volcanism, but mid-crustal rocks have not been exhumed. This difference between the amount of exhumation in south Karakorum and south Tibet could be related to the transpressive context of south Karakorum inducing a strain partitioning between the N130°E faults and east-west folding. Such partitioning produces heterogeneous uplift in this area. Moreover, zones of rapid uplift rate are associated with erosion due to the high incision rate of the large Shyok and Braldu rivers and the large Biafo-Hispar and Concordia glaciers in south Karakorum.

The Nordfjord area, north of the Hornelen Devonian basin in Western Norway, is the southernmost part of the Ultra-High Pressure (UHP) Province, defined by the occurrence of coesite-bearing and diamond-bearing continental rocks. Compilation of structural, petrological, and chronological data from the area leads to a model for the formation of dome structures at the crustal scale and the behavior of the continental crust during its exhumation from mantle depths. The Nordfjord area appears as a 100 × 50 km dome-shaped boudin affected by at least two deformation stages. A stage of E-W stretching and top-to-west shearing produced several envelopes of migmatitic gneisses bounded by narrow high-strain zones over a core preserving the Precambrian granulite protolith. This dome is affected by the west-vergent Nordfjord Mylonitic Shear Zone on its southern limb during late exhumation under the Nordfjord-Sogn Detachment Zone. The first stage of deformation is coeval with reequilibration from maximum pressure conditions around 2.8 GPa, 650 °C (THERMOCALC multiequilibrium method) in the coesite stability field to higher temperature and lower pressure conditions (1.8 GPa, 780 °C). Subsequent retrogression was recorded in the amphibolite facies (0.7 GPa, 580 °C) and in the greenschist facies (0.4 GPa, 420 °C). Dates for these stages yield exhumation velocities higher than 2 mm/yr. 40 Ar/ 39 Ar ages in the area, compared to a spectrum of cooling ages along a 500-km-long N-S profile, show that cooling of the northern part of the Western Gneiss Complex is at least 20 Ma younger than in the south. The Western Gneiss Complex is therefore the result of the late juxtaposition of two complexes, the Northwestern Gneiss Complex, characterized by UHP relics, constrictive stretching, partial melting, and doming during a multi-stage exhumation from the deep parts of the orogen, and the Southwestern Gneiss Complex with Devonian basins, a well-developed detachment system, and distinct high pressure to medium pressure units stacked together during a single and rapid exhumation stage. The two complexes may represent deep subduction channel dynamics (north) and shallower wedge circulation (south) in the Caledonian orogen. The Nordfjord Mylonitic Shear Zone appears as a major tectonic in the Western Gneiss Complex. Partial melting in the Northwestern Gneiss Complex may have favored the late exhumation of E-W elongated domes such as the Nordfjord crustal-scale boudin and their juxtaposition to the Southwestern Gneiss Complex during top-to-west shearing.

Post-orogenic extension in the Aegean Sea has produced several metamorphic domes. Some domes (“b-type”) are elongated perpendicular to the main N-S direction of extension, and they correspond to the exhumation of the middle crust along north-dipping detachments. The example of Tinos shows the progressive localization of deformation from the initial boudinage at all scales to the formation of brittle structures at the tips of boudins and the selection of one of those, which becomes the main detachment. The progressive deformation leading to strain localization is described alongside the P-T-t evolution and the role of fluid circulation. The second type of domes (“a-type”) has a long axis parallel to the direction of extension. Extension is accommodated by a detachment that exhumes high-temperature gneisses issued from deeper parts of the Hellenic edifice. Shortening perpendicular to stretching has produced the extension-parallel folds that are also observed in b-type domes but to a lesser extent. The formation of b-type and then a-type domes during extension is discussed in terms of crustal collapse during slab retreat.

This paper presents new structural data on the Naxos migmatite dome, exhumed in the central part of the Aegean Sea in Greece. The dome is cored by anatectic granites and migmatites that have preserved magmatic textures, and it is mantled by a dominantly metasedimentary sequence grading outward from amphibolite to greenschist facies. The elliptical shape of the dome is outlined by a composite transposition foliation in the mantling metasedimentary sequence. The lineation trends NNE-SSW and is associated with top-to-the-NNE shearing. Within the first order dome, kilometer-scale second order domes are evidenced by the orientation of the magmatic fabric, of the syn-migmatitic foliation trends, and by a concentration of enclaves along their margins. A network of granitic veins, structurally rooted in the migmatites, intrudes the mantling metasedimentary sequence. Subvertical granitic dikes, discordant to the foliation, are dominantly oriented parallel or perpendicular to the lineation. These dikes have preserved a magmatic texture and cross-cut partially to totally transposed veins. Kinematic analysis indicates that transposition is consistent with top-to-the-NNE shearing combined with outward rotation of the veins in the mantling metasedimentary sequence during upward migration of the migmatites in the core of the dome. Accordingly, the Naxos migmatite dome is interpreted as a diapir formed in response to a gravitational instability developed in the buoyant, partially molten rocks in a context of regional NNE-SSW extension during gravitational collapse of the Ae gean orogenic wedge.

Like the metamorphic core complexes of the Basin and Range (United States), the gneiss domes of the west European Variscan range have been associated with large-scale extension. In particular, the development of the Agly Massif, a gneiss and micaschist dome in the eastern Pyrenees (France), has been related to N-S–directed, late Variscan or Cretaceous extension. However, new microstructural and kinematic investigations in the Agly Massif demonstrate that (i) there is no major detachment, (ii) the pervasive deformation associated with the early metamorphism indicates a southward vergence, and (iii) the numerous mylonitic bands observed at different levels of the section acted as gently dipping normal faults and display opposite shear senses on both northern and southern flank of the dome. Shearing on these bands caused a multi-kilometer–scale thinning distributed across the whole lithologic column. Two new U-Pb zircon analyses yielded an age of 317 ± 3 Ma for a deformed granite from the core of the dome, and an age of 307 ± 0.4 Ma for a deformed granite emplaced in the micaschist cover. This suggests that two phases of magmatism occurred in the Agly Massif, the first prior to doming and the second during doming and the emplacement of the main Pyrenean plutons associated with a dextral transpressive phase. Therefore, the Agly gneiss dome formed in a transpressive regime and not in a late Variscan or Cretaceous extensional regime related to the collapse of a previously thickened crust.

The Sierra Nevada elongated dome in the Betic hinterland (westernmost Mediterranean region) formed by polymetamorphic, non-melted rocks involving crustal thickening and subsequent exhumation via extensional denudation including both normal faulting and vertical ductile thinning. Core rocks record a clockwise P-T-t path with segments of quasi-isothermal decompression that do not cross the melting solidi. Doming was caused by the interference of two orthogonal sets of Miocene-Pliocene, large-scale open folds (trending roughly E-W and N-S) that warp both WSW-directed extensional detachments and the footwall regional foliation. N-S folds were generated by a rolling hinge mechanism while E-W folds formed due to shortening perpendicular to the direction of extension. Strike-slip faults striking subparallel to the direction of extension laterally bound the domes, adjoining highly extended domains to less extended blocks. Using a three-dimensional model of the crustal structure of the Sierra Nevada elongated dome constrained by surface geological data, the relationships with present-day topography, and the deep crustal structure, this paper explores the role of crustal flow in the origin and evolution of the dome. Collectively, the crustal structure, the rheological considerations, and other geophysical data suggest the occurrence of flow channels at two levels: mid-crustal depths and the deep crust. Flow in the upper channel is closely related to the mode of footwall denudation by detachment unroofing. The flowing channel in the deep crust is probably induced by the NW-SE crustal thinning pattern inferred for the region, with a relatively thick crust at the NW, and is likely to be oblique to the direction of extension in the upper crust. A geometric model assuming footwall deformation by subvertical simple shear examines the possible exhumation paths of the lower-plate rocks and the evolution of the dome core in the upper crust during extension. In this model, the dome width measured parallel to the direction of extension can be used to estimate the amount of horizontal extension, once the dip of the non-readjusted segment of the detachment is well constrained. Finally, we also discuss two interesting associated problems common in extensional tectonics; namely, (1) what causes mountain uplift in recently extended continental terrains? and (2) what holds up high mountain belts in these regions where the Moho is often subhorizontal? A rolling hinge model and simultaneous transverse shortening can explain the high values of extension, the orthogonal folding, and the high mountains in the Sierra Nevada elongated dome. Flow beneath the dome of a relatively low-velocity, highly conductive, and probably low-density crustal material can support the high topography.

The Fosdick Mountains, West Antarctica, form an 80 × 15 km migmatite dome comprising massive paragneisses that exhibit polyphase fabrics, nappe-scale folds that involve granodiorite to leucogranite intrusions, and diatexite. High strain zones developed on the NE flank of the dome. Multiple generations of leucogranite sheets, dikes and diatexite intrude the dome, and evidence for partial melt in structural sites is widespread. Macroscopic folds and the maximum anisotropy of magnetic susceptibility (AMS) direction are oriented NE-SW, generally parallel with the N65W regional finite strain axis determined from brittle faults and a mafic dike array outside the dome. The direction is oblique to the inherited fault that bounds the dome, to regional trends in the surrounding Ford Ranges, and to the nearby continental margin. Paragneiss assemblages yield thermobarometry results that indicate ≥18 km depth for growth of texturally early garnet and ∼10 km depth for growth of texturally late cordierite at the expense of biotite. Nodular and dendritic forms of cordierite that develop at shallow crustal depths completely overprint dynamic fabrics. The cordierite-K feldspar-sillimanite-garnet-biotite gneisses are determined by U-Pb SHRIMP (sensitive high-resolution ion microprobe) zircon analysis to contain inherited zircon populations of 1100–1000 Ma and 500 Ma age. The U-Pb distribution is characteristic of sediments shed from the Ross Orogen of the Paleozoic Gondwana margin, represented by Swanson Formation in the Ford Ranges. A granodiorite gneiss yields 375 Ma prismatic zircon grains characteristic of Ford Granodiorite in the region. Zircon rim ages in both rock types suggest a protracted growth history during polyphase high-temperature metamorphism. The peak of metamorphism was attained at 106–99 Ma, based on prior U-Pb monazite ages and regional relationships, followed by rapid cooling through the range of 40 Ar/ 39 Ar mineral systems between 101 and 94 Ma. The timing coincides with a change from convergent to divergent tectonics along the West Gondwana margin prior to breakup. Considered together, the partial melt evidence, decompression record and rapid thermal evolution of the partially molten rocks suggests diapiric processes in effect during emplacement the Fosdick Mountains dome along the Balchen Glacier fault. The consistent NE-SW orientation of folds, AMS strain axes, stretching direction from mafic and felsic dikes, kinematic axes from minor faults, and sparse mineral lineation attest to structural controls on dome emplacement. These are interpreted as evidence of dextral transcurrent strain across the region at ca. 100 Ma.

Geologic mapping of the Kigluaik gneiss dome on Seward Peninsula, Alaska, was conducted at 1:24,000 scale and compiled with thesis maps and published maps as a colored map covering eight 15″ quadrangles at 1:63,360 scale. The Kigluaik dome consists of an exceptionally well-exposed 15-km-thick structural section of metasedimentary rocks and orthogneisses. A 91 ± 1 Ma upper-amphibolite-to-granulite facies metamorphic event that was syntectonic with gneiss dome fabrics overprints a pre–120 Ma blueschist-to-greenschist facies event. Peak conditions associated with younger metamorphism increase from greenschist facies in the structurally higher rocks of the Nome Group, through biotite, staurolite, sillimanite, and second sillimanite isograds to granulite facies in the rocks of the Kigluaik Group. The high-grade overprint is coeval with mantle-derived magmatism at 90 ± 1 Ma. Mafic magmatism provided heat for metamorphism and induced partial melting, also enabling flow of gneisses. Deformation thinned the crust as the gneiss dome was exhumed, collapsing isograds and juxtaposing rocks that equilibrated at dramatically different structural levels. Lineations in the gneiss dome record a rotation of stretching directions with structural depth. The continuous transition of lineation azimuth and the lack of overprinting relationships indicate orthogonal stretching directions at different structural levels in the gneiss dome during its formation. Based on regional geologic relations, it is likely that the Bering Strait gneiss domes developed within a continental arc that was undergoing extension, probably as the result of rollback of the north-dipping subducting slab along the Pacific margin during Late Cretaceous time. Additional mapping contributions were made by: Phillip B. Gans, Department of Geological Sciences, University of California at Santa Barbara; Andrew T. Calvert, U.S. Geological Survey, Menlo Park, California; Timothy A. Little, School of Earth Sciences, Victoria University of Wellington, New Zealand; Kimberly A. Hannula, Department of Geology, Fort Lewis College, Durango, Colorado; Jeffrey Lee, Department of Geological Sciences, Central Washington University, Ellensburg, Washington; Charles M. Rubin, Department of Geological Sciences, Central Washington University, Ellensburg, Washington; Jaime Toro, Department of Geology and Geography, West Virginia University, Morgantown, West Virginia; and James E. Wright, Department of Geology, University of Georgia, Athens, Georgia.

Gneiss domes in the Maryland Piedmont near Baltimore contain a wide variety of metamorphic rock types, all metamorphosed to amphibolite grade and collectively named Baltimore Gneiss. Most were derived from interlayered sedimentary and volcanic rocks into which granitic plutons were emplaced in both Precambrian and Paleozoic time. Migmatitic features are common in the Baltimore Gneiss. Most seem to reflect either original sedimentary layering or original layering that has been enhanced by later metamorphic differentiation, but some involved partial melting, perhaps during the Precambrian metamorphism. We find no evidence for widespread melting in rocks at the present level of exposure during the Paleozoic. Limited fluid-driven K-metasomatism may well have occurred locally, but we find no evidence for metasomatism on a regional scale. Pressure-temperature conditions for the Baltimore Gneiss during the Paleozoic metamorphism are estimated to be 7–9 kb and 625–675 °C. For half a century, the anticlinal structures exposing basement gneiss at Baltimore have been cited as classic examples of mantled gneiss domes, but more recent field studies and geophysical investigations suggest that they are the result of interference between late upright folds and early nappe structures formed by a combination of isoclinal folding along subhorizontal axial surfaces and thrust faults rooted in a regional décollement.