ABSTRACT The Coast Mountains batholith (CMB) is one of the largest continental margin batholiths in the world. It is nearly continuously exposed for >1700 km along the west coast of North America in British Columbia through southeastern Alaska into southwestern Yukon Territory. This guide, prepared for the GSA Thompson Field Forum held in August 2018, describes the geology along the readily accessible Skeena River transect of the CMB in British Columbia. At this latitude, the CMB is bounded on the east by generally low-grade stratified rocks and subordinate Jurassic to Eocene plutons. These rocks are bounded on the west by a Paleogene, low-angle, top-to-the northeast detachment (the Eastside detachment). West of the detachment, the Central Gneiss Complex (CGC), which forms the lower plate of the detachment, consists of amphibolite to granulite-facies schist, gneiss, and orthogneiss, intruded by Late Cretaceous to Paleogene plutons. The CGC is characterized by regionally consistent Eocene 40 Ar/ 39 Ar and K-Ar cooling dates. This core belt is bounded on the west by the Paleogene Coast shear zone, a steep crustal-scale structure. Paleogene plutons do not occur west of this belt. West of the Coast shear zone, schists of the Western metamorphic belt show evidence for southwest-verging thrusting, and form an inverted metamorphic sequence with grade dramatically decreasing to the west. These rocks are intruded by Jurassic to Late Cretaceous plutons. We use this transect as a basis to examine the growth of the CMB as a whole, emphasizing commonalities and variations with the batholith and how these traits may reflect magmatic processes that create this and other convergent-margin batholiths. We conclude by highlighting a few of the many open questions regarding the evolution of this complex batholith.

Originally prepared for the GSA Thompson Field Forum that ran from Terrace to Prince Rupert, British Columbia, this guide describes the geology along the Skeena River transect of the Coast Mountains batholith, the largest Cordilleran batholith of western North America and one of the largest continental-margin batholiths in the world. The last guide to this area was published in 1983 and this new volume is the only modern overview of the last decades of work. The authors use the transect as a basis to examine the growth of the Coast Mountains batholith as a whole, emphasizing commonalities and variations with the batholith and how these traits may reflect magmatic processes that create convergent-margin batholiths.

ABSTRACT New two-dimensional (2-D) thermomechanical finite-element models are used to test whether thrust advection, particularly at normal (10–20 km m.y. ‒1 ) to high (>50 km m.y. ‒1 ) horizontal slip rates, can substantially influence relatively high metamorphic heating rates (150–250 °C m.y. ‒1 ). Simple beam models that involve a single thrust with a dip of ~30° and geothermal gradients that are initially equal in the hanging wall and footwall yield maximum footwall heating rates of 15, 32, 75, and 150 °C m.y. ‒1 for imposed thrust rates of 5, 20, 50, and 100 km m.y. ‒1 (5–100 mm yr ‒1 ), respectively. Thrust rates were chosen to represent the possible range of rates interpreted in ancient collisional systems and observed in modern systems. More complex tapered wedge models, which include an elevated geothermal gradient in the hanging wall (with respect to the footwall), are intended to approximate the compressed isotherm sequences resulting from thrust-related hanging-wall exhumation predicted in previously published coupled thermomechanical models that include a strain continuum. In those models, thrust rates of 50 and 80 km m.y. ‒1 yield maximum footwall heating rates of 112 °C m.y. ‒1 and 170 °C m.y. ‒1 , respectively. In the immediate footwall of the regional-scale Ben Hope thrust in northwest Scotland, diffusion modeling of quartz inclusions in garnet yields heating rates of ~150–250 °C m.y. ‒1 . Although advective heating due to mass transfer at relatively high thrust rates cannot account for heating rates as high as those obtained from diffusion models (in Scotland and other orogens), the conduction-advection thrust models presented here suggest that thrust emplacement at relatively high rates (50–80 km m.y. ‒1 ) can contribute substantially to the total heating budget in the footwall of major thrusts. Additionally, the distribution of both footwall heating and hanging-wall cooling due to advective heat transfer along faults may have implications for the distribution of prograde and retrograde metamorphic assemblages in thrust belts. Other mechanisms that may substantially influence the thermal budget near crustal-scale faults may include shear heating, particularly at high rates of movement on thrusts, and pre- to synorogenic magma emplacement.

ABSTRACT Fold-and-thrust belts and their adjacent foreland basins provide a wealth of information about crustal shortening and mountain-building processes in convergent orogens. Erosion of the hanging walls of these structures is often thought to be synchronous with deformation and results in the exhumation and cooling of rocks exposed at the surface. Applications of low-temperature thermochronology and balanced cross sections in fold-and-thrust belts have linked the record of rock cooling with the timing of deformation and exhumation. The goal of these applications is to quantify the kinematic and thermal history of fold-and-thrust belts. In this review, we discuss different styles of deformation preserved in fold-and-thrust belts, and the ways in which these structural differences result in different rock cooling histories as rocks are exhumed to the surface. Our emphasis is on the way in which different numerical modeling approaches can be combined with low-temperature thermochronometry and balanced cross sections to resolve questions surrounding the age, rate, geometry, and kinematics of orogenesis.

ABSTRACT The structure and geology of former rifted continental margins can exert significant influence on their subsequent incorporation into collisional orogens. While thinned continental crust attached to the subducting mantle lithosphere may be incorporated into subduction channels, the weakly rifted parts of the margin are likely to resist subduction and thus deform ahead of the main orogenic front. This expectation is corroborated by a case study from the external western Alps. The former Dauphinois basins have inverted to form external basement massifs. Much of the deformation was widely distributed, with few localized thrust structures. Existing models that invoke distinct deformation events separated in time by a major (late Eocene, “Nummulitic”) unconformity are abandoned here in favor of crustal shortening that was continuous in time. Integrated stratigraphic, paleothermal, and geochronological data reveal that basin inversion was protracted over 6–10 m.y., coeval with deformation in the more internal parts of the chain. The notion of continuous, rather than episodic, deformation raises issues for the ways in which rates and tectonic activity may be evaluated within ancient orogens.

ABSTRACT The southern Appalachian western Blue Ridge preserves a Mesoproterozoic and mid-Paleozoic basement and Neoproterozoic to Ordovician rift-to-drift sequence that is metamorphosed up to sillimanite grade and dissected by northwest-directed thrust faults resulting from several Paleozoic orogenic events. Despite a number of persistent controversies regarding the age of some western Blue Ridge units, and the nature and extent of multiple Paleozoic deformational/metamorphic events, synthesis of several multidisciplinary data sets (detailed geologic mapping, geochronology and thermochronology, stable-isotope chemostratigraphy) suggests that the western Blue Ridge likely records the effects of two discrete orogenic events. The earlier Taconic (470–440 Ma) event involved a progression from open folding and emplacement of the Greenbrier–Rabbit Creek and Dunn Creek thrust sheets as a foreland fold-and-thrust to low-grade hinterland system (D 1A ), followed by deep burial (>31 km), pervasive folding of the earlier-formed fault surfaces, and widespread Barrovian metamorphism (D 1B ). Because this high-grade (D 1B ) metamorphic event is recorded in Ordovician Mineral Bluff Group turbidites, this unit must have been deposited prior to peak orogenesis, possibly as a foreland basin or wedge-top unit in front of and/or above the developing fold-and-thrust belt. The later Alleghanian (325–265 Ma) event involved widespread northwest-directed brittle thrusting and folding related to emplacement of the Great Smoky thrust sheet (D 2 ; hanging wall of the Blue Ridge– Piedmont thrust). Mid-Paleozoic 40 Ar/ 39 Ar muscovite ages from western Blue Ridge samples likely record post-Taconic cooling (hornblende and some muscovite 40 Ar/ 39 Ar ages) and/or Alleghanian thrust-related exhumation and cooling (ca. 325 Ma muscovite 40 Ar/ 39 Ar and 300–270 Ma zircon fission-track ages), as opposed to resulting from a discrete Neoacadian thermal-deformational event. The lack of evidence for a discrete Neoacadian event further implies that all deformation recorded in the Silurian–Mississippian(?) Maggies Mill–Citico Formation must be Alleghanian. We interpret this structurally isolated sequence to have been derived from the footwall of the Great Smoky fault as an orphan slice that was subsequently breached through the Great Smoky hanging wall along the out-of-sequence Maggies Mill thrust.

ABSTRACT The Berkshire massif in western Massachusetts is one of several external basement massifs in the New England Appalachians. The Day Mountain thrust is a segment of the western frontal thrust of the Berkshire massif that carried Mesoproterozoic basement gneisses and unconformably overlying cover rocks of the Neoproterozoic (?) Dalton Formation and Cambrian Cheshire Quartzite over the Cambrian to Ordovician Stockbridge Formation. The basal unit of the Dalton Formation is a distinctive deformed quartz-pebble conglomerate. We made 27 strain estimates at 18 locations using the deformed conglomerate to investigate the strain field in the Day Mountain thrust sheet and test the plane-strain model of thrust emplacement. Although the strain ellipsoids vary from prolate to oblate shapes over distances as small as 200 m, and the orientations of the principal directions of strain range widely, a remarkably simple strain pattern, broadly consistent with simple shear, emerges when the strain data are plotted on contoured stereograms. The preferred orientation of the maximum elongation direction plunges gently and approximately coincides with the west-northwest transport direction of the thrust sheet, the preferred orientation of the intermediate principal strain axis is nearly horizontal and perpendicular to the transport direction, and the preferred orientation of the short axis plunges steeply. Most of the strain ellipsoids fall in the prolate field, which is indicative of constrictive flow, especially in the northern part of the thrust sheet. We suggest that the steep gradients in three-dimensional strain type were caused by flow of the more ductile conglomerate over an irregular surface of relatively rigid basement rocks, which were little affected by Paleozoic deformation. The constrictive flow conditions that dominate the strain field in the northern part of the thrust sheet may reflect the irregular paleotopography of the unconformity surface and/or a lateral ramp oriented at an oblique angle to the transport direction that impeded west-northwest–directed thrusting.

ABSTRACT Final closure of the Neotethys Ocean basin along the Eurasian margin in southeastern Europe during Eocene–Oligocene time was accompanied by upper-crustal extension expressed as a series of low-angle detachments, basins bounded by normal faults, and volcanism. This extensional belt spanned the southern Balkan Peninsula from the Albanides along the southern Adriatic coast in the west to western Anatolia in the east. Despite the widespread occurrence of this phenomenon within the southern Balkan region, similar extension has not previously been observed in association with the Neotethys closure in the Dinarides, which form the western geographic continuation of this orogenic belt, ending in the Austrian Alps in the northwest. The Mid- Bosnian Schist Mountains are a fault-bounded body of greenschist-facies metamorphic rocks located along the paleogeographic margin of the present-day Adria continental block in the Internal Dinarides. We combine low-temperature thermochronometric ages with field observations of kinematic shear sense indicators and demonstrate that the Mid- Bosnian Schist Mountains were exhumed along a normal fault between 43 and 27.5 Ma. The most rapid cooling occurred between ca. 35 and 27 Ma, coincident with a regional-scale magmatic event. These data constitute the first evidence for major extension in the Dinarides contemporaneous with collision between Adria and the Eurasian margin, and they are consistent with removal of a subducting slab during the transition between oceanic subduction and continental collision.

ABSTRACT Midcrustal channel flow has been hypothesized to be responsible both for the Greater and Lesser Himalayan Sequences (the Miocene Himalayan channel theory), and for the present eastward and northward movement and extension of the Tibetan upper crust (the Tibetan middle-crustal channel flow theory). Because processes within the crust cannot be directly observed, various studies have attempted to validate midcrustal channel flow theory by using indirect approaches, including outcrop patterns and other field data from the Himalayas, Tibet, and exposed older orogenic roots. The results have been highly debated, because arguments can be made that the internal structure of a channel and, therefore, the outcrop patterns of a paleomidcrustal channel are not unique. This paper investigates the types of structural patterns that may be produced within a midcrustal channel and discusses why they can be difficult, if not impossible, to distinguish from outcrop patterns produced by other mechanisms. A new example from the exposed middle crust of southern Finland is also discussed in this context. While outcrop structural patterns must indeed agree with other potential results that may imply a midcrustal channel, the inverse is not necessarily true: One cannot infer a midcrustal channel based on outcrop patterns alone, due to the nonunique nature of the patterns.

ABSTRACT Much of the early prograde history in metamorphic rocks is lost due to overprinting at near-peak conditions or through retrograde modification during exhumation. Fortunately, inclusions encapsulated in rigid porphyroblasts may preserve a record of early burial conditions. Quartz inclusions in garnet porphyroblasts from the Strafford Dome, eastern Vermont, have homogeneous Ti concentrations ([Ti]) that differ from matrix quartz, which retains a history of Si-liberating metamorphic reactions and fluid influx. We applied growth-composition models to evaluate potential processes associated with Ti partitioning in quartz before encapsulation in garnet, including a model for constant-volume growth of quartz due to mineral dissolution-transfer processes and growth as a result of Si-liberating diagenetic and metamorphic reactions. Because these processes typically occur at low temperatures, quartz with exceedingly low [Ti] (<<1 ppm) would be formed and cannot account for the homogeneous Ti distribution at concentrations between 2.5 and 5 ppm observed in the sample. This suggests that chemical reequilibration through dynamic recrystallization must have taken place prior to encapsulation in garnet. Analysis of fluid and graphite inclusions with Raman spectroscopy in different microstructural settings allowed the characterization of fluid composition and temperature of microstructure development early in the prograde history. The findings from this study exemplify the utility of garnet hosts to shield inclusion minerals from chemical modification and recrystallization during later events. As such, they provide a window into the early stages of orogenesis and provide insights concerning the mechanisms controlling equilibration of quartz.

ABSTRACT The Gyeonggi Massif, Korea, consists of basement gneisses and supracrustal rocks migmatized to varying degrees. We conducted a petrologic-geochronologic study of the Mount Cheonggye gneisses, located in the western part of the Gyeonggi Massif, and we discuss the crustal evolution of the massif based on our results combined with a compilation of available data from the literature. Mineral assemblages and reaction textures in cordierite-garnet-biotite gneisses suggest a composite pressure-temperature path defined by two clockwise trajectories, M 1 and M 2 . Pseudosection modeling constrains M 1 peak metamorphic conditions as ~10.5 kbar and 840– 860 °C, followed by M 2 recrystallization at 4.5–5.5 kbar and 720–770 °C. Textural relationships of garnet to cordierite and kyanite to plagioclase transitions, as well as the pseudosection analysis, corroborate the clockwise pressure-temperature-time paths in the Gyeonggi Massif. We dated the polyphase metamorphism using sensitive high-resolution ion microprobe (SHRIMP) U-Pb data for zircon and monazite grains from eight samples. Overgrowth rims of zircon in a cordierite-garnet-biotite gneiss and a K-feldspar megacrystic orthogneiss yielded weighted mean 207 Pb/ 206 Pb ages of 1854 ± 9 Ma ( n = 11) and 1852 ± 12 Ma ( n = 19), respectively. This Paleoproterozoic age was reproduced by monazite grains from three cordierite-bearing gneisses dated at ca. 1861–1851 Ma. In contrast, monazite grains from a cordierite-bearing mylonitic gneiss and two biotite gneisses yielded consistent 206 Pb/ 238 U ages ranging from 235 ± 2 Ma ( n = 12) to 231 ± 2 Ma ( n = 15), suggesting a strong Triassic thermal overprint. Finally, we dated a postkinematic granitic dike at ca. 226 Ma, suggesting Late Triassic termination of the orogenesis. Our compilation of SHRIMP U-Pb ages from zircon, monazite, allanite, and titanite available from the literature confirms that the Gyeonggi Massif underwent two distinct thermal events in association with Paleoproterozoic (1.88–1.85 Ga) and Triassic (245–230 Ma) collisional orogenies. In contrast, Mesoproterozoic to Paleozoic thermal episodes are present in the Gyeonggi marginal belt, newly named in this study, where Neoproterozoic (ca. 950–750 Ma) and Paleozoic (ca. 450–430 Ma) ages are prominent in magmatic and detrital zircons. Our tectonic model, exemplified by the Qinling-Gyeonggi microcontinent, suggests that prolonged accretionary tectonics produced arc-related lithologies overlying the Gyeonggi Massif basement rocks. The juxtaposition of these terranes onto the Gyeonggi Massif produced tectonic mixtures with affinities to either the North or South China cratons. On the basis of similarities in zircon age distributions, we further suggest that the Qinling-Gyeonggi microcontinent is built upon basement rocks with North China craton affinity, at least in the Korean Peninsula and extending toward the Japanese Islands.

ABSTRACT Ion microprobe U-Pb zircon rim ages from 39 samples from across the accreted terranes of the central Blue Ridge, eastward across the Inner Piedmont, delimit the timing and spatial extent of superposed metamorphism in the southern Appalachian orogen. Metamorphic zircon rims are 10–40 µm wide, mostly unzoned, and dark gray to black or bright white in cathodoluminescence, and truncate and/or embay interior oscillatory zoning. Black unzoned and rounded or ovoid-shaped metamorphic zircon morphologies also occur. Th/U values range from 0.01 to 1.4, with the majority of ratios less than 0.1. Results of 206 Pb/ 238 U ages, ±2% discordant, range from 481 to 305 Ma. Clustering within these data reveals that the Blue Ridge and Inner Piedmont terranes were affected by three tectonothermal events: (1) 462–448 Ma (Taconic); (2) 395–340 Ma (Acadian and Neoacadian); and (3) 335–322 Ma, related to the early phase of the Alleghanian orogeny. By combining zircon rim ages with metamorphic isograds and other published isotopic ages, we identify the thermal architecture of the southern Appalachian orogen: juxtaposed and superposed metamorphic domains have younger ages to the east related to the marginward addition of terranes, and these domains can serve as a proxy to delimit terrane accretion. Most 462–448 Ma ages occur in the western and central Blue Ridge and define a continuous progression from greenschist to granulite facies that identifies the intact Taconic core. The extent of 462–448 Ma metamorphism indicates that the central Blue Ridge and Tugaloo terranes were accreted to the western Blue Ridge during the Taconic orogeny. Zircon rim ages in the Inner Piedmont span almost 100 m.y., with peaks at 395–385, 376–340, and 335–322 Ma, and delimit the Acadian-Neoacadian and Alleghanian metamorphic core. The timing and distribution of metamorphism in the Inner Piedmont are consistent with the Devonian to Mississippian oblique collision of the Carolina superterrane, followed by an early phase of Alleghanian metamorphism at 335–322 Ma (temperature >500 °C). The eastern Blue Ridge contains evidence of three possible tectonothermal events: ~460 Ma, 376–340 Ma, and ~335 Ma. All of the crystalline terranes of the Blue Ridge–Piedmont megathrust sheet were affected by Alleghanian metamorphism and deformation.

ABSTRACT Growth of continental crust requires addition of juvenile material from the mantle and/or oceanic lithosphere. Large-scale addition is most likely to occur by accretion of oceanic arcs and continental arc magmatism. Therefore, evaluation of fluxes of new crustal material into continents requires testing for crustal recycling by melting of older crust and reincorporation of continental sediments into continental arcs. This work uses isotopic data and pressure-temperature-time ( P-T-t ) paths to evaluate the juvenile sedimentary contribution to crustal growth versus evolved sedimentary rocks recycled from preexisting continental crust in the Cretaceous Cascades magmatic arc. Neodymium isotope ratios for 91–75 Ma Swakane Gneiss metasedimentary rocks are compatible with a significant proportion of recycled crustal material, with ε t Nd values ranging from 2.1 to -5.3 and a preponderance of values less than 0.7. Clockwise metamorphic P-T-t paths consist of four segments: (1) initial loading and heating to garnet growth conditions, (2) near-isothermal loading during early garnet growth, (3) near-isobaric heating to 650–710 °C and 8–11 kbar, and (4) decompression and cooling. The transition from isothermal loading to isobaric heating is interpreted to reflect thrust loading and associated subsequent heating. Similarities in the P-T-t paths for all samples require a similar tectonic mechanism to produce the high- P before high- T metamorphic history. We conclude that high-grade rocks of the Swakane Gneiss preserve petrologic evidence for synorogenic deposition, metamorphism that resulted from heating caused by overthrusting, and subsequent exhumation. These results demonstrate the importance of thrust loading in magmatic arcs as a mechanism for growth and recycling of continental crust.