Cameron Davidson

Ductile extrusion of the Higher Himalayan Crystalline in Bhutan: evidence from quartz microfabrics

Quartz textures measured from deformed quartz tectonites within the Lesser Himalaya and Higher Himalaya Crystalline of Bhutan show similar patterns. Orientation and distribution of the quartz crystallographic axes were used to confirm the regional shear sense: the asymmetry of c-axis and a-axis patterns consistently indicates top-to-the-south shearing. The obliquity of the texture and the inferred finite strain (plane strain to moderately constrictional), suggest the strain regime had a combination of rotational and irrotational strain path. In most of the samples from the Bhutan Himalaya, the inferred deformation mechanisms suggest moderate- to high-temperature conditions of deformation that produced the observed crystallographic preferred orientation. Much higher temperature of deformation is indicated in the quartz veins from a leucogranite. n nThe observed ductile deformation is pervasively developed in the rocks throughout the investigated area. The intensity of deformation increases only slightly in the vicinity of the Main Central Thrust. Simultaneous southward shearing within a large part of the Higher Himalaya Crystalline near and above the Main Central Thrust and normal faulting across the South Tibetan Detachment, is explained by the tectonically induced extrusion of a ductily deforming wedge. The process of extrusive flow suggested here can be approximated quantitatively by channel flow models that have been used to describe subduction zone processes. Channel flow accounts for some observed phenomena in the Himalayan orogen such as inverted metamorphic sequences near the Main Central thrust, not related to an inversion of isotherms, and the syntectonic emplacement of leucogranites into the extruding wedge, locally leading to an inversion of isotherms due to heat advection.

U-Th-Pb geochronology of the Coast Mountains batholith in north-coastal British Columbia: Constraints on age and tectonic evolution

Previously published and new U-Pb geochronologic analyses provide 313 zircon and 59 titanite ages that constrain the igneous and cooling history of the Coast Mountains batholith in north-coastal British Columbia. First-order findings are as follows:n (1) This segment of the batholith consists of three portions: a western magmatic belt (emplaced into the outboard Alexander and Wrangellia terranes) that was active 177–162 Ma, 157–142 Ma, and 118–100 Ma; an eastern belt (emplaced into the inboard Stikine and Yukon-Tanana terranes) that was active ca. 180–110 Ma; and a 100–50 Ma belt that was emplaced across much of the orogen during and following mid-Cretaceous juxtaposition of outboard and inboard terranes. (2) Magmatism migrated eastward from 120 to 80 (or 60) Ma at a rate of 2.0–2.7 km/Ma, a rate similar to that recorded by the Sierra Nevada batholith. (3) Magmatic flux was quite variable through time, with high (>35–50 km 3 /Ma per km strike length) flux at 160–140 Ma, 120–78 Ma, and 55–48 Ma, and magmatic lulls at 140–120 Ma and 78–55 Ma. (4) High U/Th values record widespread growth (and/or recrystallization) of metamorphic zircon at 88–76 Ma and 62–52 Ma. (5) U-Pb ages of titanite record rapid cooling of axial portions of the batholith at ca. 55–48 Ma in response to east-side-down motion on regional extensional structures. (6) The magmatic history of this portion of the Coast Mountains batholith is consistent with a tectonic model involving formation of a Late Jurassic–earliest Cretaceous magmatic arc along the northern Cordilleran margin; duplication of this arc system in Early Cretaceous time by >800 km (perhaps 1000–1200 km) of sinistral motion (bringing the northern portion outboard of the southern portion); high-flux magmatism prior to and during orthogonal mid-Cretaceous terrane accretion; low-flux magmatism during Late Cretaceous–Paleocene dextral transpressional motion; and high-flux Eocene magmatism during rapid exhumation in a regime of regional crustal extension.

Mesozoic and Cenozoic tectonics of the eastern and central Alaska Range: Progressive basin development and deformation in a suture zone

Analysis of late Mesozoic and Cenozoic sedimentary basins, metamorphic rocks, and major faults in the eastern and central Alaska Range documents the progressive development of a suture zone that formed as a result of collision of an island-arc assemblage (the Wrangellia composite terrane) with the former North American continental margin. New basin-analysis, structural, and geochronologic data indicate the following stages in the development of the suture zone: (1) Deposition of 3–5 km of Upper Jurassic–Upper Cretaceous marine strata (the Kahiltna assemblage) recorded the initial collision of the island-arc assemblage with the continental margin. The Kahiltna assemblage exposed in the northern Talkeetna Mountains represents a Kimmeridgian–Valanginian backarc basin that was filled by northwestward-flowing submarine-fan systems that were transporting sediment derived from Mesozoic strata of the island-arc assemblage. The Kahiltna assemblage exposed in the southern Alaska Range represents a Valanginian–Cenomanian remnant ocean basin filled by west-southwestward–flowing submarine-fan systems that were transporting sediment derived from Paleozoic continental-margin strata uplifted in the along-strike suture zone. A belt of retrograde metamorphism and a regional anticlinorium developed along the continental margin from 115 to 106 Ma, roughly coeval with the end of widespread deposition in the Kahiltna sedimentary basins. (2) Metamorphism of submarine-fan deposits of the Kahiltna ba sin, located near the leading edge of the island-arc assemblage, occurred at ca. 74 Ma, as determined from a new U-Pb zircon age for a synkinematic sill. Coeval with metamorphism of deposits of the Kahiltna basin in the southern part of the suture zone was development of a thrust-top basin, the Cantwell basin, in the northern part of the suture zone. Geologic mapping and compositional data suggest that the 4 km of Upper Cretaceous nonmarine and marginal marine sedimentary strata in this basin record regional subaerial uplift of the suture zone. (3) Shortening and exhumation of the suture zone peaked from 65 to 60 Ma on the basis of metamorphic and geochronologic data. In the southern part of the suture zone, submarine-fan deposits of the Kahiltna basin, which had been metamorphosed to kyanite schists at ∼25 km depth and ∼650 °C, were exhumed and cooled through the biotite closure temperature (∼300 °C) by ca. 62 Ma. In the northern part of the suture zone, this time period was marked by shortening, uplift, and erosion of sedimentary strata of the Cantwell basin. (4) From 60 to 54 Ma, ∼3 km of volcanic strata were deposited over deformed sedimentary strata of the Cantwell basin, and several granitic plutons (the McKinley sequence) were emplaced along the suture zone. (5) Following igneous activity, strike-slip displacement occurred from ca. 54 to 24 Ma along the Denali fault system, which had developed in the existing suture zone. Late Eocene–Oligocene strike-slip displacement resulted in the formation of several small sedimentary basins along the Denali fault system. (6) Regional transpressive shortening characterized the suture zone from ca. 24 Ma to the present. Flexural subsidence, related to regional shortening, is represented by late Eocene to Holocene nonmarine deposits of the Tanana foreland basin. Regional subsidence resulted in Miocene coal seams up to 20 m thick and well-developed lacustrine deposits. Overlying the Miocene deposits are ∼1.2 km of Pliocene and Holocene conglomeratic deposits. Compositional and paleocurrent data from these younger deposits record regional Neogene uplift of the suture zone and recycling of detritus from older basins to the south that had become incorporated into the uplifted suture zone. Geologic mapping of major thrust faults along the northern and southern margins of the suture zone documents Paleozoic strata thrust over both Pliocene fluvial deposits and Quaternary glacial deposits of the Tanana basin. These mapping relationships provide evidence that regional shortening continues to the present in the eastern and central Alaska Range.

Role of melt in the formation of a deep-crustal compressive shear zone: The MaClaren Glacier Metamorphic Belt, south central Alaska

The Maclaren Glacier metamorphic belt is an exhumed portion of a deep-crustal shear zone where hot, upper amphibolite facies rocks were emplaced over cooler, lower grade rocks. It is located in south central Alaska within the collision zone between terranes previously accreted to North America and the Wrangellia superterrane. Crosscutting relationships and orientations of thin granitoid sills within the hanging wall show that melt was repeatedly intruded into the shear zone during overall compression. In addition, a 1 km thick tonalire sill (the Valdez Creek tonalire) was emplaced into the shear zone while it was active; this conclusion is based on the presence of a well-developed foliation within the sill which is concordant to the fabric of the shear zone, the alignment and tiling of plagioclase laths, the presence of highly strained mafic enclaves within a matrix which shows no evidence of any subsolidus deformation, and the preservation of a delicate magmatic texture which indicates that a magmatic fluid was still present after deformation in the sill had ceased. A one-dimensional thermal model for sill-shaped plutons of varying thicknesses, which are emplaced into country rocks at different initial temperatures, indicates that melt can be present for long periods of time in the deep crust (> 1 m.y.). An important factor controlling the length of time that a sill remains molten is the temperature of the surrounding rocks which, in the lower crust, can be as high as the solidus temperature of granitoid melts. If the amount of time for melt to crystallize is sufficiently long, potentially large amounts of strain will be accommodated by zones containing melt due to the low strength of melt compared to rock. In the Maclaren Glacier metamorphic belt, the amount of time needed for the Valdez Creek tona!ite to fully crystallize is calculated to be about 90,000 years. This is a minimum value since the original thickness of the sill is not known. Nevertheless, using the 90,000 year value and assuming that the bulk of convergence between North America and Wrangellia was concentrated within the Valdez Creek tonalire, a displacement of at least !0 km could have been accommodated across the sill while melt was present.

KINEMATICS AND TECTONIC SIGNIFICANCE OF TRANSPRESSIVE STRUCTURES WITHIN THE COAST PLUTONIC COMPLEX, BRITISH COLUMBIA

Abstract Structural data from the Coast Plutonic Complex, near Prince Rupert, British Columbia, are consistent with a deformational history dominated by dextral transpression from Campanian to Paleocene time. Penetrative east-side-up, southwest-directed, ductile shearing produced moderately northeast-plunging overturned kilometer-scale isoclinal folds. These folds are dextrally sheared and refolded into kilometer-scale upright northwest-plunging folds and steeply dipping transposed foliations with moderate to shallow northwest-plunging lineations along their western side. An east-side-up component to the transcurrent shearing is kinematically compatible with east-side-up shearing found within the Great Tonalite Sill. Kinematic and geometric gradients and the spatial distribution of the finite stretching direction are interpreted to result from partitioning of transpression. The location of these structures and overprinting relationships suggest the Great Tonalite Sill intruded late-kinematically into a crustal-scale dextral transpressive shear zone. These results indicate this shear zone could form part of the Baja-B.C. fault system that would have accommodated large northward displacements of the terranes making up western British Columbia and southeast Alaska. This conclusion is based on: (1) It is favorably located to accommodate the proposed displacements; (2) Deformation occurred during the time period of proposed large displacement (83–59xa0Ma); (3) The 15-km thickness of the shear zone indicates it records large displacements.

Symmagmatic folding of the base of the Bergell pluton, Central Alps

Abstract Evidence for magmatic, submagmatic and solid-state deformation in tonalite, granodiorite and country rocks found at the deep-seated floor (22–26 km) of the Bergell pluton demonstrates that final emplacement and crystallization occurred during regional deformation of the pluton and the underlying country rocks. After northward emplacement over the country rocks, but before complete crystallization, the floor of the pluton was folded during simultaneous N-S shortening and E-W stretching. This is evidenced by synmagmatic folds with E-W striking, nearly vertical axial planes, and by regional east-plunging stretching lineations in the country rocks which are parallel to the regional-scale fold axes and the magmatic mineral lineations in the pluton. Opposite senses of shear from the well-foliated, occasionally mylonitic contact suggest that deformation was mostly accomplished by pure shear. Synmagmatic deformation is related to late-stage N-S shortening of the Alpine orogen and shows that the still partially molten pluton responded to low differential stress very much like the country rocks deformed in the solid-state at high temperatures. Post-emplacement tilting associated with backthrusting along the Insubric mylonites led to the exposure of the plutons floor at its present-day western margin.

Paleomagnetism and geochronology of the Ecstall pluton in the Coast Mountains of British Columbia: Evidence for local deformation rather than large‐scale transport

[1]xa0Samples for geochronologic, geobarometric, and paleomagnetic analyses were collected across the northern portion of the Ecstall pluton southeast of Prince Rupert, British Columbia. Al-in-hornblende geobarometry indicates pressures from 740 ± 10 to 840 ± 30 MPa corresponding to crystallization depths of ∼25 to ∼30 km. U/Pb analyses of zircons from western, central, and eastern localities within the pluton yield crystallization ages of 91.5 ± 1.0 Ma, 90.8 ± 1.0 Ma, and 90.5 ± 1.0 Ma, respectively. Rock magnetic experiments, reflected light microscopy, and thermal demagnetization behavior suggest that natural remanent magnetism is carried by low-Ti titanohematite. Unblocking temperatures of the characteristic remanent magnetization (ChRM) are dominantly in the 560°C to 630°C range, with age of magnetization approximated by the 40Ar/39Ar hornblende ages of 84.2 ± 0.10 Ma on the western margin and 76.4 ± 0.6 Ma in the center of the pluton. Site-mean ChRM directions were isolated for paleomagnetic samples from 23 sites and are distributed along a small circle with subhorizontal axis at ∼340° azimuth. ChRM directions from the central portion of the pluton are concordant with the expected Cretaceous magnetic field direction, while ChRM directions from the western margin are discordant by >70°. Folding of the Ecstall pluton, either during Late Cretaceous west directed thrust transport above the convex upward Prince Rupert Shear Zone or during younger deformation of the pluton and underlying shear zone, can account for the paleomagnetic data and is consistent with the geochronologic, geobarometric, and structural geologic observations.

SOUTHWESTERN LAURENTIAN ZIRCONS IN UPPER CRETACEOUS FLYSCH OF THE CHUGACH-PRINCE WILLIAM TERRANE IN ALASKA

The Chugach-Prince William (CPW) terrane in southern Alaska is dominated by thick imbricated flysch, mainly Maastrichtian to Paleocene, that represents one of the thickest accretionary complexes in the world. Detrital zircons from sandstones from across the belt are dominated by grains with crystallization ages close to the age of deposition, and hence the source region supported a long-lived Late Cretaceous to Paleocene volcanic arc. The metaplutonic basement that supported this arc was made of rocks with Mesozoic zircons (Jurassic), with a minor fraction of Paleozoic (Devonian) and Precambrian grains. There is continuity of grain ages across the belt from the Shumagin Islands in the west to the eastern localities, including the Yakutat Group. Positive εHf isotopic ratios in zircons (from the arc and basement) are consistent with melting of a relatively juvenile source, and the leading source candidate is the Coast Plutonic Complex that intrudes the Wrangellia composite terrane. The small fraction of Precambrian detrital zircons, typically <5 percent, reveal two cohorts with distinct histories. A western cohort of Precambrian grains from Shumagins, Kodiak, and Prince William Sound have grain age populations and isotopic signatures consistent with a northern Laurentian source. These grains have a wide range of ages, but are dominated by populations between 1810 to 1870 Ma and 2520 to 2680 Ma. Collectively these grains have εHf (t) from +13.9 to –21.1, but most grains >1800 Ma have negative εHf (t) values consistent with an evolved source. Raman spectroscopy on zircons from this suite indicates total radiation damage and internal disorder is considerable and comparable to 0.5 to 1.0 Gyr of accumulated damage. Thus the source rocks had to have resided high in the crustal column for most of the Phanerozoic. An eastern cohort of Precambrian grains have grain-ages and an isotopic signature consistent with a southern Laurentian source – mainly Yavapai-Mazatzal, Mojave and the Granite-Rhyolite province of the southwest United States. These zircons are dominated by populations at ∼1380 Ma, ∼1485 Ma, and ∼1722 Ma and εHf (t) values are mostly positive, ranging from +11.7 to –3.4. All zircons between 1436 and 1716 Ma have positive εHf (t) values, which indicate juvenile source rocks, a unique and distinctive aspect of SW Laurentia. Total radiation damage and internal disorder in this suite of detrital grains is minimal and comparable to ∼100 Myr of accumulated damage, which requires source rocks to have been heated to amphibolite grade in the Cretaceous. Thus the low radiation damage may require derivation from in and around the arc, and not farther inland. We hypothesize that the majority of the flysch of the CPW terrane accumulated in an accretionary complex flanking what is now the Coast Plutonic Complex, but that zircons from the eastern cohort of the CPW were derived from rocks of the Sierran-Southern California-Peninsular Arc, which collapsed along a narrow tectonic corridor due to the subduction of the conjugate to the Shatsky Plateau in the Maastrichtian – the Mojave breach. This point source of southwestern Laurentian rocks allowed sediment with a distinct Precambrian signature to spill out into the forearc and thus part of the Chugach flysch shares a common history with part of the Franciscan Complex. We hypothesize that some of the Chugach flysch was deposited in a trench adjacent to this southwestern Laurentian point source and was subsequently displaced ∼3200 km northward to Alaska.

Age, origin, and significance of brittle faulting and pseudotachylyte along the Coast shear zone, Prince Rupert, British Columbia

Northwest-striking brittle faults containing cataclasite, fault gouge, and, in one location, pseudotachylyte are common in the vicinity of the Coast shear zone near Prince Rupert, British Columbia. The pseudotachylyte locality, found in a dextral strike-slip fault zone along the Coast shear zone, contains spherulites, alkali feldspar microlites, and amygdules, suggesting that the pseudotachylyte crystallized rapidly from a melt phase within 5 km of the surface. 4 0 Ar/ 3 9 Ar incremental heating of the pseudotachylyte matrix yielded a weighted mean plateau of 29.8 ′ 0.6 Ma and an inverse isochron of 29.8 ′ 1.5 Ma with an 4 0 Ar/ 3 6 Ar intercept of 296.0 ′ 15.2. These results show that pseudotachylyte associated with brittle faulting can be dated precisely, and imply that some dextral coast-parallel displacement occurred across the Coast shear zone in the Oligocene and that the majority of exhumation in the Coast Mountains at the latitude of Prince Rupert (∼54°N) was accomplished by 30 Ma.

Provenance of Quartz Arenites of the Early Paleozoic Midcontinent Region, USA

Quartz arenites characterize much of the early Paleozoic sedimentary history of the midcontinent region. Despite numerous studies, the century-long debate on how these arenites formed is still unresolved, primarily because of the compositional and textural purity of the deposits. In this study, we present an extensive data set of detrital zircon geochronology from the early Paleozoic supermature arenites of the midcontinent region, and we offer new constraints about their origin. Our results coupled with compiled provenance information from older basins and orogens may indicate that the Cambrian and Ordovician arenites represent sediment reworking primarily of two different older basins. The Cambro-Ordovician sediment was transported to the midcontinent region by two early Paleozoic river systems that sourced from the paleo-east (Huron basin) and paleo-northeast (midcontinent rift region).