Multiple sediment incorporation events in a continental magmatic arc: Insight from the metasedimentary rocks of the northern North Cascades, Washington (USA)

The rheology and composition of arc crust and the overall evolution of continental magmatic arcs can be affected by sediment incorporation events. The exhumed Cretaceous–Eocene North Cascades arc exposes abundant meta­ sedimentary rocks that were incorporated into the arc during multiple events. This study uses field relationships, detrital zircon geochronology, bulk rock geo­ chemistry, geothermometry, and quartz­in­garnet geobarometry to distinguish approximate contacts and emplacement depths for different metasedimentary units to better understand their protolith incorporation history and impact on the arc. The Skagit Gneiss Complex is one of the main deep crustal units of the North Cascades arc. It includes metasedimentary rocks with distinct detrital zircon signatures: Proterozoic–Cretaceous (Group 1) or Triassic–Cretaceous (Group 2) zircon populations. Both metasedimentary groups achieved near­ peak metamorphic conditions of 640–800 °C and 5.5–7.9 kbar; several Group 2 samples reveal the higher pressures. A third group of metasedimentary rocks, which was previously interpreted as metamorphosed equivalents of backarc sediments (Group 3), exhibited unimodal Triassic or bimodal Late Jurassic– Early Cretaceous detrital zircon signatures and achieved near­peak conditions of 570–700 °C and 8.7–10.5 kbar. The combined field and analytical data indi­ cate that protoliths of Group 1 and Group 2 metasedimentary rocks were successively deposited in a forearc basin and underthrusted into the arc as a relatively coherent body. Group 3 backarc sediments were incorporated into the arc along a transpressional step­over zone. The incorporation of both forearc and backarc sediments was likely facilitated by arc magmatism that weakened arc crust in combination with regional transpression

Finishing the euchromatic sequence of the human genome

The sequence of the human genome encodes the genetic instructions for human physiology, as well as rich information about human evolution. In 2001, the International Human Genome Sequencing Consortium reported a draft sequence of the euchromatic portion of the human genome. Since then, the international collaboration has worked to convert this draft into a genome sequence with high accuracy and nearly complete coverage. Here, we report the result of this finishing process. The current genome sequence (Build 35) contains 2.85 billion nucleotides interrupted by only 341 gaps. It covers ∼99% of the euchromatic genome and is accurate to an error rate of ∼1 event per 100,000 bases. Many of the remaining euchromatic gaps are associated with segmental duplications and will require focused work with new methods. The near-complete sequence, the first for a vertebrate, greatly improves the precision of biological analyses of the human genome including studies of gene number, birth and death. Notably, the human enome seems to encode only 20,000-25,000 protein-coding genes. The genome sequence reported here should serve as a firm foundation for biomedical research in the decades ahead

Marketing Review Quiz: An Academic Application of the Popular Quiz Game Format

Adapting the popular quiz game format for use in the classroom can increase student involvement in marketing courses. Game preparation and play are described in detail, and both student and instructor benefits are presented.Yeshttps://us.sagepub.com/en-us/nam/manuscript-submission-guideline

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 degrees C to 400-500 degrees C, and structural data indicate pure shear-dominant (W-m <= 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 similar to 20%-110% at 2-5 km above the MCT and between similar to 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 similar to 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 large-scale, distributed structural thinning, and highlights the diverse range of processes that accommodate tectonic denudation during orogenesis

Does Underthrusting Crust Feed Magmatic Flare‐Ups in Continental Arcs?

Abstract Episodic magmatic flare‐ups are documented in many continental arcs worldwide. Yet, the causes of such episodicity and the sources feeding the flare‐ups are not well‐understood. In this study, we use a 1‐D numerical model and scaling analysis to assess the mass balance and thermodynamic feasibility of generating arc magma as a result of partial melting of underthrusted retro‐arc lower crust. Results show the magma volumetric flux or magmatic thickening rate, is directly correlated with the crustal underthrusting rate and the relative timescales of heat transfer and underthrusting. For a continental arc with dimensions similar to the Sierra Nevada arc in California, we show with a constant underthrusting rate of 5 km/Myr, the magmatic thickening rate is 0.1–0.3 km/Myr. This is slightly below the baseline of arc magma thickening rate (∼0.3 km/Myr) from the mantle wedge and accounts for 10%–30% of the magmatic thickening rate during a flare‐up. The cumulative volume of magma generated from the partial melting of a 20‐km‐thick underthrusted lower crust is on the order of 105 km3, about 10%–40% of the estimated magma volume generated during a flare‐up. Therefore, we argue partial melting of underthrusted lower crust plays a partial or subsidiary role in driving a magmatic flare‐up event. Additional melts from the mantle and/or other crustal sources are needed to achieve the observed magmatic output during flare‐ups. The arc root developed by partial melting of the underthrusted crust reduces the time needed to obtain the critical thickness for root foundering, thus influencing the tempo of arc magmatism

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 similar to 400-500 degrees C to similar to 700-750 degrees C between 675m below and 200m above the MCT. This defines a 269 +/- 44 degrees C/km inverted gradient, interpreted to have formed via high-magnitude (similar to 100-250 km) shearing on a discrete MCT zone delineated by the limits of inverted metamorphism. Temperatures increase from similar to 300-400 degrees C to similar to 400-530 degrees 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% to 36-49%, respectively, between 2.3 and 1.0 km below the MCT. The MCT acted as a "stretching fault," with translation on the order of hundreds of kilometers accompanied by transport-parallel stretching of footwall and hanging wall rocks on the order of tens of kilometers. This demonstrates that stretching accommodated between major shear zones can make a significant contribution to cumulative mass transfer

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 arcsand 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 melange 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 melange 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 margin-parallel translation from as far as the latitude of southern California

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 epsilon(Hf) 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 accretionary-wedge 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

Fabric development during exhumation from ultrahigh-pressure in an eclogite-bearing shear zone, Western Gneiss Region, Norway

Petrofabrics and trace-element thermobarometry of deformed quartzofeldspathic gneiss and associated coesite-bearing eclogite in the Salt Mylonite Zone (Western Gneiss Region, Norway) document a pressure–temperature–deformation path from ultrahigh-pressure to amphibolite-facies conditions. The Salt mylonite zone is dominated by quartzofeldspathic gneiss with a strong foliation and lineation. Coesite-bearing eclogite within the shear zone contains a foliation and lineation (defined by elongate omphacite) consistent with that of the host gneiss, suggesting that gneiss and eclogite were deformed in the same kinematic framework. In eclogite, omphacite preserves LS- to L-type crystallographic preferred orientation, and quartz preserves prism fabrics that developed in quartz near coesite–quartz transition conditions. The quartzofeldspathic gneiss in the mylonite zone records prism and rhomb slip in quartz and reverse zoning in plagioclase (higher Ca rims) consistent with re-equilibration during decompression. The Ti concentration in quartz in gneiss is higher than that in quartz in eclogite, suggesting that quartz recrystallized at a lower pressure in the gneiss. Ti-in-quartz thermobarometry of rutile-bearing eclogite and titanite-bearing gneiss indicates equilibration at T > 750 °C and T < 650 °C, respectively. This mylonite zone preserves a discontinuous record of fabric development from incipient stages of exhumation of ultrahigh-pressure rocks to crustal conditions.13 page(s

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 Mathe youngest orthogneisses overlap Ar-40/Ar-39 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 degrees-310 degrees versus similar to 320 degrees, 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 similar to 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