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In Marie Byrd Land, West Antarctica, the Fosdick Mountains migmatite-cored gneiss dome was exhumed from mid- to lower middle crustal depths during the incipient stage of the West Antarctic Rift system in the mid-Cretaceous. Prior to and during exhumation, major crustal melting and deformation included transfer and emplacement of voluminous granitic material and numerous intrusions of mantle-derived diorite in dikes. A succession of melt- and magma-related structures formed at temperatures in excess of 665 ± 50 °C based on Ti-in-zircon thermometry. These record a transition from wrench to oblique extensional deformation that culminated in the development of the oblique South Fosdick Detachment zone. Solid-state fabrics within the detachment zone and overprinting brittle structures record translation of the detachment zone and dome to shallow levels. To determine the duration of exhumation and cooling, we sampled granite and gneisses at high spatial resolution for U–Pb zircon geochronology and 40 Ar/ 39 Ar hornblende and biotite thermochronology. U–Pb zircon crystallization ages for the youngest granites are 102 Ma. Three hornblende ages are 103 to 100 Ma and 12 biotite ages are 101 to 99 Ma. All overlap within uncertainty. The coincidence of zircon crystallization ages with 40 Ar/ 39 Ar cooling ages indicates cooling rates > 100 °C/m.y. that, when considered together with overprinting structures, indicates rapid exhumation of granite and migmatite from deep to shallow crustal levels within a transcurrent setting. Orientations of structures and age-constrained crosscutting relationships indicate counterclockwise rotation of stretching axes from oblique extension into nearly orthogonal extension with respect to the Marie Byrd Land margin. The rotation may be a result of localized extension arising from unroofing and arching of the Fosdick dome, extensional opening within a pull-apart zone, or changes in plate boundary configuration. The rapid tectonic and temperature evolution of the Fosdick Mountains dome lends support to recently developed numerical models of crustal flow and cooling in orogenic crust undergoing extension/transtension, and accords with numerous studies of migmatite-cored gneiss domes in transcurrent settings.

The relationship between microstructure and fluid flow traced by hydrogen isotope ratios (δD) is examined within the Wildhorse detachment system of the Pioneer metamorphic core complex in south-central Idaho. Within the detachment footwall, 100-m-thick mylonitic quartzite containing minor white mica and K-feldspar displays a NW-trending stretching lineation and consistent top-to-the-NW sense of shear criteria. Microstructures within the detachment footwall comprise two groups: quartz ribbons and relict quartz grains flattened within the foliation, with porphyroclastic white mica fish; and intensely deformed and recrystallized quartz with high-aspect-ratio white mica arranged within C′ shear bands. White mica δD values are highly negative and cluster around −145‰ in high-aspect-ratio white mica and around −120‰ in porphyroclastic white mica fish. The most negative values are interpreted to reflect interaction with meteoric fluids from a high-elevation catchment (3000−4000 m), and the less negative values are interpreted to represent incomplete hydrogen isotope exchange between the meteoric fluid and the pre-extensional metamorphic fluid δD values in the white mica porphyroclasts. A suite of tightly clustered 40 Ar/ 39 Ar ages from synkinematic white mica in the detachment footwall dates deformation, recrystallization, fluid-rock interaction, and therefore the presence of high topography at 38−37 Ma; these ages are consistent with the cooling/exhumation history of the high-grade core of the Pioneer metamorphic core complex in the late Eocene. The 38−37 Ma 40 Ar/ 39 Ar ages are substantially younger than previously published ages of high topography in British Columbia to the north (49−47 Ma), in line with the hypothesis that high topography propagated from north to south in the northern segment of the North American Cordillera through Eocene time.

A diorite pluton and widely distributed mafic dykes occur in the Fosdick migmatite–granite complex, which is interpreted to represent middle-to-lower crustal rocks of the paleo-Pacific active continental margin of Gondwana. The mafic dykes exhibit a variety of relationships with host rocks in the field ranging from undeformed dykes with sharp contacts with host gneisses to dismembered dykes with commingled textures and numerous back-veins of leucosome intruded from host migmatitic gneisses suggestive of significant interaction with crustal rocks. New U–Pb ages for magmatic zircon in these rocks yield Cretaceous crystallization ages ranging from ca 113 Ma to ca 98 Ma for the mafic dykes and ca 100 Ma for the diorite pluton. These mafic intrusive rocks, which contain abundant hydrous minerals, are medium- to high-K-series calc-alkaline rocks with basic–intermediate compositions (47–59 wt.% SiO2 for the mafic dykes and 52–56 wt.% SiO2 for the diorite pluton). They have trace element patterns characterized by LILE enrichments and negative Nb anomalies indicating an origin from a hydrous mantle source metasomatized by slab-derived components. The samples without evidence of interaction with crustal rocks (11 of 14 samples), which are likely to better reflect the mantle source composition, have positive eSr(100 Ma) values (+ 8.1 to + 14.5) and negative to slightly positive eNd(100 Ma) values (− 1.6 to + 2.5) consistent with derivation from an enriched mantle source. These eleven samples may be divided into two groups either characterized by higher LILE/HFSE ratios, less radiogenic eSr(100 Ma) values and more radiogenic eNd(100 Ma) values, or characterized by relatively lower LILE/HFSE ratios, more radiogenic eSr(100 Ma) values and less radiogenic eNd(100 Ma) values suggesting differences in the mantle source. The results of this study are consistent with the melting of a variably metasomatized sub-arc mantle source during a transition from a wrench to a transtensional tectonic setting, but are inconsistent with a mantle plume origin.

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 eHf 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 eHf 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.

40–20 Ma marks a fundamental interval in the evolution of the 70–0 Ma Panamanian magmatic arc system. During this period, there is no evidence of Panamanian magmatic arc activity to the east of the Panama Canal Basin while to the west and in localized regions to the east of the Panama Canal Basin a phase of intrusive-only activity is recorded. Fundamentally, geochemical and geochronological evidence presented herein indicate that this intrusive activity was predominantly ‘adakitic-like’ and becomes younger from west to east along an approximately W–E striking lineament. Granodiorites of the Petaquilla batholith, western Panama yield LAM-ICP-MS 206 Pb/ 238 U zircon ages of 29.0 + 0.7, − 0.6 Ma, 28.5 + 0.7, − 0.5 Ma, 28.3 + 0.5, − 0.4 Ma and 26.2 + 0.5, − 0.9 Ma. To the east of the Panama Canal Basin zircons from a hypabyssal diorite of the mainly intermediate Maje subvolcanic suite, cedes a mean 206 Pb/ 238 U age of 18.9 + 0.4 Ma. Relative to other 70–5 Ma Panamanian magmatic arc lavas and intrusives, Maje and Petaquilla intrusives yield adakitic-like major and trace element abundances (e.g., > 15 wt.% Al 2 O3, generally > 3.5 wt.% Na 2 O, > 400 ppm Sr, 120) and strongly fractionated HREE patterns. These 30–26 Ma (Petaquilla) and 19 Ma (Maje) suites are also compositionally similar to a subvolcanic suite of rare, circa 25 Ma adakitic-like, andesitic intrusives which occur within the Panama Canal Basin midway between Petaquilla and Maje and at the same approximate latitude as Petaquilla and Maje. Collectively, the geochemical and geochronological data for the adakitic-like intrusives arc consistent with formation via partial melting of lowermost, mafic crust above a sub-horizontal slab tear that propagated from the west (Petaquilla) to the east (Maje) between 30 and 19 Ma. Our new tectonic model postulates that collision between the Panamanian magmatic arc system and an ‘indentor’ (e.g., a tract of thickened buoyant, oceanic crust or plateau) occurred at about 40 Ma, a time of which coincides with the initiation of left-lateral offset of the Early (i.e., 70–40 Ma) Arc system. This collision resulted in the shutdown of the Early Arc system, possible steepening of the subducting Farallon slab and ultimately slab break-off and the phase of mainly adakitic-like intrusive activity. Subsequent to slab removal by ~ 20 Ma, NE-dipping subduction jumped to the south and initiated production of the Later (i.e., post 20 Ma) Arc system soon thereafter.

The rise of the Isthmus of Panama, linked to a number of climatic, paleoceanographic, and biological events, has been studied mostly from indirect, often distal, geochemical and biotic evidence. We have upgraded existing geologic mapping in central Panama with more than 2000 field stations, over 40 petrographic analyses, and more than 30 new geochronological and thermochronological analyses. This data set suggests that the isthmus was an uninterrupted chain above sea level from late Eocene until at least late Miocene times. The basement complex of central Panama is a folded-faulted, ∼3-km-thick arc sequence, intruded by granitoid bodies and onlapped by mildly deformed upper Eocene and Oligocene strata. Six U/Pb zircon ages in the granitoids–along with published geochronological data—reveal intense late Paleocene to middle Eocene magmatism (58–39 Ma), a temporary cessation of magmatic activity between 38 and 27 Ma, and renewed magmatism between 25 and 15 Ma in a position ∼75 km south of the former magmatic axis. Thermochronological analyses in zircon (eight U-Th/He ages), and in apatite crystals (four U-Th/He ages and nine fission-track ages) obtained from a subset of 58–54 Ma granitoid bodies record a concordant Lutetian-age (47–42 Ma) cooling from ∼200 °C to ∼70 °C in ∼5 m.y., and cooling below ∼40 °C between 12 and 9 Ma. Cooling is linked to exhumation by an angular unconformity that separates the deformed basement complex below from mildly deformed, upper Eocene to Oligocene terrestrial to shallow-marine strata above. Exhumation and erosion of the basement complex are independently confirmed by lower Miocene strata that have a detrital zircon signature that closely follows the central Panama basement complex age distribution. These results greatly restrict the width and depth of the strait separating southern Central America from South America, and challenge the widely accepted notion that the Central American Seaway closed in late Pliocene time, when the ice age began.

The upward transfer of partially molten crust and the formation of gneiss domes and metamorphic core complexes commonly take place by localization of normal or oblique extension in the middle and upper crust. In Marie Byrd Land, Antarctica, a transition from wrench to oblique extension occurred during oblique plate divergence along the East Gondwana margin and intracontinental crustal extension associated with the West Antarctic Rift System in mid-Cretaceous time. Migmatites in the Fosdick dome record steep fabrics formed during wrenching, and associated granite networks display crystallization ages of 117–115 Ma. These steep fabrics are overprinted by subhorizontal foliation and leucogranite sheets with crystallization ages in the 109–102 Ma range. Syntectonic emplacement of granite sheets in the South Fosdick detachment zone indicates that detachment tectonics led to rapid exhumation of the terrain by 100 Ma. This study has implications for understanding melt transport, magma accumulation, and the formation of detachments in an oblique tectonic setting.

[1] The Fosdick Mountains, West Antarctica, expose a 15 x 80 km migmatite-cored gneiss dome consisting of migmatitic gneisses, diatexite migmatite, and subhorizontal leucogranite sheets. The Fosdick dome was emplaced and exhumed in the mid-Cretaceous due to oblique extension associated with the West Antarctic Rift system along the West Antarctic–New Zealand segment of East Gondwana. The dome is bounded to the south by a dextral oblique detachment structure and to the north by an inferred dextral strike-slip fault. Within the Fosdick dome and in the detachment zone, granite occupies leucosomes, dikes, sills, and dilatant and shear structures. The pattern of kilometer-scale domains of migmatite and granite suggest that lithologic variations and heterogeneous deformation (boudinage) resulted in pressure gradients that enhanced melt flow and magma accumulation in the Fosdick dome. Steep foliations are overprinted, folded, and transposed by subhorizontal fabrics. The crosscutting relationship is interpreted as a transition from wrench deformation to oblique divergence. Steep structures in the dome host concordant, subvertical leucosome and granite sheets yielding SHRIMP U-Pb zircon ages between ca. 117 and 114 Ma. Prevalent subhorizontal domains host large volumes of subhorizontal diatexite migmatite and granite sheets that yield U-Pb zircon ages between ca. 109 and 102 Ma. These ages indicate a timescale for melt influx of approximately 15 Ma and that the transition from wrench to oblique divergence may have occurred in as little as 5 Ma. Granites with crystallization ages between ca. 109 and 102 Ma were also emplaced in the South Fosdick Detachment zone, indicating that the detachment was active during oblique divergence. SHRIMP U-Pb titanite ages between ca. 102 and 97 Ma for late- to post-tectonic diorite dikes are interpreted as emplacement ages and give a minimum age for gneissic foliation development during detachment faulting. The Fosdick Mountains preserve a record of the middle to lower crustal response to a transition from wrench to oblique extensional deformation. Overprinting structural relationships show that a change in the angle of oblique extension can induce accumulation of subhorizontal magma sheets and lead to initiation of a detachment zone.

A group of plutons were emplaced in the western Klamath Mountains province during the waning stages of the Late Jurassic Nevadan orogeny. Published U-Pb (zircon) ages indicate that the “western Klamath plutonic suite” was emplaced in the age range of 151‐144 Ma. Crosscutting relationships, development of contact metamorphic aureoles, and the presence of distinctive inherited zircon populations indicate that the magmas intruded the footwall and hanging-wall rocks of the principal Nevadan thrust fault. The plutons are chiefly gabbroic to dioritic in composition, but commonly include ultramafic rocks and contain smaller volumes of tonalite and granodiorite. Hornblende is the most common mafic phase, except for some ultramafic rocks in which clinopyroxene ± olivine are locally distinctive, the two-pyroxene dioritic to monzodioritic rocks of the Buck Lake unit of the Bear Mountain pluton, and the most felsic rocks in which biotite is the most abundant mafic phase. Compositions of fine-grained mafic dikes suggest the presence of two principal parental, H 2 O-rich magmas: primitive basalt and evolved basalt/basaltic andesite. The former was parental to the ultramafic rocks of this suite. It was also parental to the basalt/basaltic andesite magmas by deep-seated fractional crystallization pro