Stacia M. Gordon

Time Scales of Metamorphism, Deformation, and Crustal Melting in a Continental Arc, North Cascades USA

The record of metamorphism, deformation, and melting in the North Cascades continental arc provides insights into the timing and mechanisms of extensional unroofing that followed crustal thickening. The Skagit Gneiss (North Cascades) is composed of variably deformed 90–45 Ma tonalitic to granitic intrusive rocks. These lithologies and metasedimentary rocks are migmatitic. Zircon and monazite from different textural varieties of migmatite from three outcrops along an east-west transect were analyzed using the isotope dilution–thermal ionization mass spectrometry (ID-TIMS) technique. The data reveal two main migmatization pulses: (1) 68–63 Ma and (2) 53–47 Ma. In the westernmost locality, leucosome zircon yields group 1 dates, and mesosome (sillimanite-garnet-biotite gneiss) zircon is as young as 61 Ma. Leucosome zircon in the easternmost outcrop yields only group 2 dates, and biotite gneiss contains 47 Ma zircon. The third outcrop has leucosome dates from both groups: 65 Ma and 53 Ma. Monazite from leucosomes and mesosomes likely records the timing of prograde metamorphism (ca. 69 Ma) and later synkinematic ± fluid-mediated growth (49–46 Ma). These results indicate that partial melting occurred during 71–61 Ma metamorphism and lasted through 46 Ma deformation associated with exhumation. The results and field observations suggest the presence of ductilely deforming crust from 68 to 46 Ma. The similarity between the youngest U-Pb dates in the migmatites and cooling and basin-filling ages suggests a link between ductile and brittle processes over a range of structural levels in the arc, from a zone of crustal melting, flow, and migmatite crystallization to Earth9s surface.

Temperature and strain gradients through Lesser Himalayan rocks and across the Main Central thrust, south central Bhutan: Implications for transport‐parallel stretching and inverted metamorphism

In order to understand mass and heat transfer processes that operated during Himalayan orogenesis, we collected temperature, finite and incremental strain, and kinematic vorticity data through a 5 km thickness of Lesser and Greater Himalayan rocks in southern Bhutan. This transect crosses two major shear zones, the Main Central thrust (MCT) and Shumar thrust (ST). Raman spectroscopy on carbonaceous material and garnet-biotite thermometry are integrated with deformation temperatures from quartz petrofabrics. These data define inverted field gradients that correspond in structural position with the MCT and ST, which are separated by sections in which temperatures remain essentially constant. Temperatures increase from ~400-500 °C to ~700-750 °C between 675 m below and 200 m above the MCT. This defines a 269 ± 44 °C/km inverted gradient, interpreted to have formed via high-magnitude (~100-250 km) shearing on a discrete MCT zone delineated by the limits of inverted metamorphism. Temperatures increase from ~300-400 °C to ~400-530 °C across the ST, which is attributed to differences in maximum burial depth of hanging wall and footwall rocks. Strain and vorticity data indicate that Lesser and Greater Himalayan rocks were deformed by layer-normal flattening. Transport-parallel lengthening and foliation-normal shortening increase from 38-71% and 36-49%, respectively, between 2.3-1.0 km below the MCT. The MCT acted as a ‘stretching fault’, with translation on the order of 100s of km accompanied by transport-parallel stretching of footwall and hanging wall rocks on the order of 10s of km. This demonstrates that stretching accommodated between major shear zones can make a significant contribution to cumulative mass transfer.

Linking deep and shallow crustal processes during regional transtension in an exhumed continental arc, North Cascades, northwestern Cordillera (USA)

The North Cascades orogen (northwestern USA) provides an exceptional natural laboratory with which to evaluate potential temporal and kinematic links between processes operating at a wide range of crustal levels during collapse of a continental arc, and particularly the compatibility of strain between the upper and lower crust. This magmatic arc reached a crustal thickness of ≥55 km in the mid-Cretaceous. Eocene collapse of the arc during regional transtension was marked by magmatism, migmatization, ductile flow, and exhumation of deep crustal (8–12 kbar) rocks in the Cascades crystalline core coeval with subsidence and rapid deposition in nonmarine basins adjacent to the core, and intrusion of dike complexes. The Skagit Gneiss Complex is the larger of two regions of exhumed deep crust with Eocene cooling ages in the Cascades core, and it consists primarily of tonalitic orthogneiss emplaced mainly in two episodes of ca. 73–59 Ma and 50–45 Ma. Metamorphism, melt crystallization, and ductile deformation of migmatitic metapelite overlap the orthogneiss emplacement, occurring (possibly intermittently) from ca. 71 to 53 Ma; the youngest orthogneisses overlap 40 Ar/ 39 Ar biotite dates, compatible with rapid cooling. Gently to moderately dipping foliation, subhorizontal orogen-parallel (northwest-southeast) mineral lineation, sizable constrictional domains, and strong stretching parallel to lineation of hinges of mesoscopic folds in the Skagit Gneiss Complex are compatible with transtension linked to dextral-normal displacement of the Ross Lake fault zone, the northeastern boundary of the Cascades core. The other deeply exhumed domain, the 9–12 kbar Swakane Biotite Gneiss, has a broadly north-trending, gently plunging lineation and gently to moderately dipping foliation, which are associated with top-to-the-north noncoaxial shear. This gneiss is separated from overlying metamorphic rocks by a folded detachment fault. The Eocene Swauk and Chumstick basins flank the southern end of the Cascades core. In the Swauk basin, sediments were deposited in part at ca. 51 Ma, folded shortly afterward, and then covered by ca. 49 Ma Teanaway basalts and intruded by associated mafic dikes. Directly after dike intrusion, the fault-bounded Chumstick basin subsided rapidly. Extension directions from these dikes and from Eocene dikes that intruded the Cascades core are dominantly oblique to the overall trend of the orogen (275°–310° versus ∼320°, respectively) and to the northwest-southeast to north-south ductile flow direction in the Skagit and Swakane rocks. This discordance implies that coeval extensional strain was decoupled between the brittle and ductile crust. Strain orientations at all depths in the Cascades core contrast with the approximately east-west extension driven by orogenic collapse in coeval metamorphic core complexes ∼200 km to the east. Arc-oblique to arc-parallel flow in the Cascades core probably resulted in part from dextral shear along the plate margin and from along-strike gradients in crustal thickness and temperature.

Evolution of the Jura-Cretaceous North American Cordilleran margin: Insights from detrital-zircon U-Pb and Hf isotopes of sedimentary units of the North Cascades Range, Washington

The U-Pb age and Hf-isotope composition of detrital zircons from Jurassic to Upper Cretaceous sedimentary rocks adjacent to the southern North Cascades–Coast Plutonic Complex continental magmatic arc document shifting provenance, the tectonic evolution of the arc system, and translation along the continental margin. Systematic changes in the detrital-zircon data provide insight that the western margin of North America evolved from: marginal basins adjacent to continent-fringing oceanic arcs (ca. 160–140 Ma); forearc basins adjacent to mid-Cretaceous (ca. 120–90 Ma) Andean-type continental arcs; and addition of a cratonic source to forearc and accretionary wedge units to Cordilleran arc systems in the mid-Late Cretaceous (ca. 85 Ma). Jurassic Methow terrane, Nooksack Formation, and western mélange belt units dominantly contain detrital zircons derived from accreted oceanic terranes, whereas Lower Cretaceous strata from the same units have age peaks that correspond to known pulses of magmatism in Cordilleran continental magmatic arc systems. The age peaks and Hf-isotope signature of the Jurassic and Lower Cretaceous strata are comparable to multiple sources exposed along the margin. In contrast, the Upper Cretaceous western mélange belt has distinct Precambrian zircon populations and unradiogenic Late Cretaceous zircons that are more similar to southwestern than northwestern Laurentian sources. Statistical comparisons confirm provenance similarities between rocks of the North Cascades and those 700–2000 km to the south and, thus, support marginparallel translation from as far as the latitude of southern California.

Distributed north-vergent shear and flattening through Greater and Tethyan Himalayan rocks: Insights from metamorphic and strain data from the Dang Chu region, central Bhutan

In several places in the Himalaya, there are debates over the location of and defining criteria for the South Tibetan detachment (STD) system. Here, we attempt to resolve this debate in central Bhutan by interpreting temperature, pressure, finite strain, and shear-sense data from an 11-km-thick structural transect through the Dang Chu region. Raman spectroscopy on carbonaceous material and garnet-biotite thermometry define a gradual, structurally upward decrease from 600–700 °C to 400–500 °C, and structural data indicate pure shear-dominant (Wm ≤0.4), layer-normal flattening strain and north-vergent shearing distributed through most of the section. Our data, when combined with published data from central Bhutan, define gradual, structurally upward cooling and an upright pressure gradient that is 1.2–2.4 times lithostatic distributed between 0 and 11 km above the Main Central thrust (MCT). Transport-parallel lengthening varies between ~20%–110% at 2–5 km above the MCT and between ~5%–55% at 5–11 km above the MCT, and north-vergent shearing is distributed between 2 and 11 km above the MCT. These data rule out the presence of a discrete, normal-sense shear zone and instead illustrate distributed structural thinning accommodated by north-vergent shearing. The strain data allow for ~85 km of distributed north-vergent displacement, which may be related to differential southward transport during MCT emplacement. Alternatively, distributed shear may have been translated northward into the STD system in northern Bhutan. Timing constraints for shearing on the MCT and STD allow for both possibilities. Central Bhutan provides a case study for largescale, distributed structural thinning, and highlights the diverse range of processes that accommodate tectonic denudation during orogenesis. LITHOSPHERE; v. 9; no. 5; p. 774–795; GSA Data Repository Item 2017271 | Published online 14 July 2017 https://doi.org/10.1130/L655.1

Rapid time scale of Earth’s youngest known ultrahigh-pressure metamorphic event, Papua New Guinea

Phengite eclogite (PNG08-010f), collected from the coesite locality on Tumabaguna Island, consists mainly of elongate garnet and omphacite that define a weak foliation. The overall mineral-phase proportions preserved in PNG08-010f are ~47% omphacite, ~34% garnet ~13% quartz, ~3% phengite, ~2% rutile, and <<0.5% amphibole and epidote. Interstitial quartz and minor phengite, with accessory phases apatite, rutile, and zircon, are found as inclusions and in textural equilibrium with garnet and omphacite. Minor symplectite growth of diopsidic clinopyroxene and/or amphibole and albite occurs along rims of garnet, omphacite, and phengite. Rare amphibole, ferro-taramite–taramite, is preserved as inclusions in garnet and locally is associated with breakdown reactions forming muscovite, quartz, plagioclase, and alunite. The well-preserved UHP peak assemblage consists of garnet + omphacite + coesite/quartz + phengite + rutile. Garnet contains inclusions of phengite, rutile, zircon, and rare amphibole. The subhedral to anhedral garnets are typically 0.1–0.5 mm across and exhibit a very limited range of compositions: Alm58–61Prp22–24Grs16–19Sps1 (Table DR1). Omphacite is subhedral to anhedral, typically 0.8 mm across, has sodic-rich compositions (Jd63–68), and occurs as the main matrix phase. Larger omphacite grains (3.5 mm in length) are also found in the sample and have the same composition. Phengite occurs as tabular grains that are typically 0.7 mm in length and some rims show minor retrogression to fine-grained biotite and plagioclase. The grains are weakly zoned, with a decrease in celadonite component from core to rim. In addition, the phengite show a range in Si from 3.33–3.51 atoms per formuls unit (a/fu), but typical matrix grains have Si = 3.33–3.43 a/fu (Table DR1). Quartz occurs within the matrix and as inclusions within garnet and adjacent omphacite that are associated with distinct radial fractures. Zircons extracted from the fresh matrix of this eclogite contain inclusions of the peak assemblage, including omphacite, garnet, rutile, and rare phengite (FIG. 2A). The ~5–10 μm inclusions of omphacite and garnet have identical compositions to matrix omphacite (Jd66) and garnet (Alm59Prp23Grs18Sps1) (Table DR1), indicating zircon growth during peak metamorphic mineral crystallization from ca. 6.0–5.2 Ma.

Provenance and metamorphism of the Swakane Gneiss: Implications for incorporation of sediment into the deep levels of the North Cascades continental magmatic arc, Washington

The Swakane Gneiss, interpreted to represent sedimentary strata metamorphosed at 8–12 kbar, is the deepest exposed crustal levels within the exhumed North Cascades continental magmatic arc, yet the nature and age of its protolith and the mechanism by which it was transported to deep-crustal levels remains unclear. Zircons from 11 paragneiss and schist samples were analyzed for U-Pb age and Hf-isotope composition in order to investigate the tectonic history of the Swakane Gneiss from protolith deposition to metamorphism within the North Cascades arc. Zircons interpreted to have crystallized in situ during metamorphism and/or melt-crystallization within the Swakane Gneiss at depth have ca. 74–66 Ma ages. Detrital-zircon age and Hf-isotope characteristics demonstrate provenance shifts that correlate with maximum depositional ages of ca. 93–81 Ma. Samples deposited between ca. 93 and 88 Ma have dominantly Mesozoic age peaks with initial εHf values between depleted mantle and chondritic uniform reservoir (CHUR), whereas ca. 86–81 Ma sample show the addition of distinct Proterozoic populations (ca. 1380 and 1800–1600 Ma) and Late Cretaceous zircons with unradiogenic Hf-isotope compositions. Similar detrital-zircon age and Hf-isotope patterns are observed in several Upper Cretaceous forearc and accretionary wedge units between southern California and Alaska along the North American continental margin. The connection between the Swakane Gneiss and these potential protoliths located outboard of Cordilleran arc systems indicate burial by either underplating of accretionarywedge sediments or underthrusting of forearc sediments. Therefore, the protolith and incorporation history for the Swakane Gneiss is likely similar to those of deep crustal metasedimentary units elsewhere in the North Cascades (i.e., the Skagit Gneiss Complex) and to the south along the continental margin (i.e., the Pelona-Orocopia-Rand schists and Schist of Sierra de Salinas). These observations suggest that burial of sediment outboard of continental magmatic arc systems may be a major mechanism for the transfer of sediment to the deep levels of continental arcs. LITHOSPHERE; v. 10; no. 3; p. 460–477; GSA Data Repository Item 2018144 | Published online 3 April 2018 https://doi.org/10.1130/L712.1

Using eclogite retrogression to track the rapid exhumation of the Pliocene Papua New Guinea UHP Terrane

The D’Entrecasteaux Islands of eastern Papua New Guinea (PNG) host the youngest known ultrahigh-pressure terrane on Earth and represent the only location where ultrahigh-pressure (UHP) rocks have been exhumed in an active rift. The PNG (U)HP rocks, consisting of Pliocene eclogites, garnet amphibolites and migmatitic gneisses, are exposed in five domal structures across the Islands. Zirconium-in-rutile thermometry records peak temperatures of 780 C from the eastern Oiatabu and nearby central Mailolo Domes, and hotter temperatures of 825–865 C within the western Goodenough Dome. Uranium–lead (U–Pb) and trace element zircon compositions from a suite of eclogite, host gneiss, felsic dikes and pegmatite from three domes document the rapid exhumation history of the PNG UHP terrane. High-spatial resolution laser-ablation split-stream inductively coupled plasma-mass spectrometry (LASS ICP-MS) analyses of select eclogite zircons exhibit no resolvable age zoning within single crystals. The same eclogite zircons, combined with separate zircons extracted from additional eclogite, host gneiss and felsic intrusions, were subsequently analysed by high-precision U–Pb chemical-abrasion isotope-dilution thermal ionization mass spectrometry and solution ICP-MS trace element analysis (TIMS-TEA). The results record discrete tectonic events across the three domes at sub-million year timescales: (1) (re)crystallization of host gneiss within the lower crust exposed in the eastern Oiatabu Dome from c.5 7–4 5 Ma; (2) initial retrogression and local decompression melting of eclogites from the Oiatabu and Mailolo Domes at c.4 6–4 3 Ma; (3) melt crystallization of weakly deformed felsic dikes of the Oiatabu Dome at c.3 0–2 9 Ma; and (4) retrogression and melt crystallization within eclogite–amphibolite-facies rocks in the western Goodenough Dome at c.2 9–2 6 Ma. In comparison to Zr-in-rutile peak temperature estimates, Ti-in-zircon temperatures >800 C may reflect increased temperatures during exhumation that resulted in partial melting of the eclogites. Inclusions of crystallized hydrous melt consisting of Na-rich plagioclase 6 K-feldspar þ quartz within eclogite zircons document this process. The elevated temperatures and the presence of the polyphase inclusions are the first documentation of partial melting of the (U)HP eclogites within PNG during initial retrogression from c.4 6–4 3 Ma. Overall, U–Pb zircon geochronology and geochemistry track both the timing of retrogressive overprinting within the lower-to-middle crust and final upper crustal emplacement over a VC The Author(s) 2018. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com 2017 J O U R N A L O F P E T R O L O G Y Journal of Petrology, 2018, Vol. 59, No. 10, 2017–2042 doi: 10.1093/petrology/egy088 Advance Access Publication Date: 18 September 2018

Transfer of Metasupracrustal Rocks to Midcrustal Depths in the North Cascades Continental Magmatic Arc, Skagit Gneiss Complex, Washington: Sediment Burial in the North Cascades

The metasupracrustal units within the north central Chelan block of the North Cascades Range, Washington, are investigated to determine mechanisms and timescales of supracrustal rock incorporation into the deep crust of continental magmatic arcs. Zircon U-Pb and Hf-isotope analyses were used to characterize the protoliths of metasedimentary and metaigneous rocks from the Skagit Gneiss Complex, metasupracrustal rocks from the Cascade River Schist, and metavolcanic rocks from the Napeequa Schist. Skagit Gneiss Complexmetasedimentary rocks have (1) a wide range of zircon U-Pb dates from Proterozoic to latest Cretaceous and (2) a more limited range of dates, from Late Triassic to latest Cretaceous, and a lack of Proterozoic dates. Two samples from the Cascade River Schist are characterized by Late Cretaceous protoliths. Amphibolites from the Napeequa Schist have Late Triassic protoliths. Similarities between the Skagit Gneiss metasediments and accretionary wedge and forearc sediments in northwestern Washington and Southern California indicate that the protolith for these units was likely deposited in a forearc basin and/or accretionary wedge in the Early to Late Cretaceous (circa 134–79Ma). Sediment was likely underthrust into the active arc by circa 74–65 Ma, as soon as 7 Ma after deposition, and intruded by voluminous magmas. The incorporation of metasupracrustal units aligns with the timing of major arc magmatism in the North Cascades (circa 79–60 Ma) andmay indicate a link between the burial of sediments and pluton emplacement.