Magma Loading in the Southern Coast Plutonic Complex, British Columbia and Washington

The southen end of the 1800 km long Coast Plutonic Complex (CPC), exposed in the Harrison Lake area of British Columbia and in the North Cascades of Washington, bears a record of great crustal thickening -20 to 40 km in localized zones during Late Cretaceous times. During this period, the CPC was positioned at the continental margin during collision/subduction of the Farallon plate. Arc magmatism and regional orogenic contraction were both active as potential crustal thickening processes. Magmatism is favored in this report as the dominant factor based on the delineation of four spatially and temporally separate loading events, the close association of the loaded areas with emplacement of large plutons, and a paucity of evidence of deep regional tectonic contraction. The timing and spatial location of crustal loading events are documented by the following: zircon ages in plutons; an early event of low pressure in pluton aureoles evidenced by andalusite, now pseudomorphed by high-pressure minerals; high pressures in country rock in pluton aureoles measured by mineral compositions in the assemblages garnet-biotite-muscovite-plagioclase and garnet-aluminum silicate-plagioclase; high pressures recorded in plutons by Al-in-hornblende barometry; and uplift ages of plutons derived from K-Ar and Ar-Ar ages of micas and hornblende in plutons.

The southern Coast Mountains, British Columbia: New interpretations from geological, seismic reflection, and gravity data

The southern Coast Mountains of British Columbia are characterized by voluminous plutonic and gneissic rocks of mainly Middle Jurassic to Eocene age (the Coast Plutonic Complex), as well as metamorphic rocks, folds, and thrust and reverse faults that mostly diverge eastward and westward from an axis within the present mountains, and by more localized Eocene and younger normal faults. In the southeastern Coast Mountains, mid-Cretaceous and younger plutons intrude Bridge River, Cadwallader, and Methow terranes and overlap Middle Jurassic through Early Cretaceous marine clastic rocks of the Tyaughton–Methow basin. The combination of geological data with new or reanalyzed geophysical data originating from Lithoprobe and related studies enables revised structural interpretations to be made to 20 km depth. Five seismic profiles show very cut-up and chaotic reflectivity that probably represents slices and segments of different deformed and rearranged rock assemblages. Surface geology, seismic interpretations, physical properties, and gravity data are combined in two profiles across the Coast Mountains to generate two new 2-D density models that are interpreted in terms of the geological units. The western part of the southern Coast Mountains consists primarily of Jurassic to mid-Cretaceous plutons to depths of 20 km with slices of Wrangellia (in the west) and Early Cretaceous volcanic and sedimentary rocks (Gambier group) in the upper 10 km. The eastern part, east of the Owl Creek fault, consists of slices of Cadwallader and Bridge River terranes and Tyaughton–Methow basin strata with limited slices of plutonic rocks at depths less than 10 km. Below that, Eocene and Late Cretaceous plutons dominate for another 10 km.

Obducted nappe sequence in the San Juan Islands – northwest Cascades thrust system, Washington and British Columbia

The San Juan Islands – northwest Cascades thrust system in Washington and British Columbia is composed of previously accreted terranes now assembled as four broadly defined composite nappes stacked on a continental footwall of Wrangellia and the Coast Plutonic Complex. Emplacement ages of the nappe sequence are interpreted from zircon ages, field relations, and lithlogies, to young upward. The basal nappe was emplaced prior to early Turonian time (∼93 Ma), indicated by the occurrence of age-distinctive zircons from this nappe in the Sidney Island Formation of the Nanaimo Group. The emplacement age of the highest nappe in the thrust system postdates 87 Ma detrital zircons within the nappe. The nappes bear high-pressure – low-temperature (HP–LT) mineral assemblages indicative of deep burial in a thrust wedge; however, several features indicate that metamorphism occurred prior to nappe assembly: metamorphic discontinuities at nappe boundaries, absence of HP–LT assemblages in the footwall to the nappe pile, and absence of significant unroofing detritus in the Nanaimo Group. A synorogenic relationship of the thrust system to the Nanaimo Group is evident from mutually overlapping ages and by conglomerates of Nanaimo affinity that lie within the nappe pile. From the foregoing relations, and broader Cordilleran geology, the tectonic history of the nappe terranes is interpreted to involve initial accretion and subduction-zone metamorphism south of the present locality, uplift and exhumation, orogen-parallel northward transport of the nappes as part of a forearc sliver, and finally obduction at the present site over the truncated south end of Wrangellia and the Coast Plutonic Complex.

Eocene rotation of the Coast Plutonic Complex and Intermontane Belt: paleomagnetism of Eocene plutons along the Klondike Highway

Paleomagnetic results are reported for the ∼59 Ma Skagway, ∼54 Ma Fraser, ∼53 Ma Summit Lake, and ∼48 Ma Clifton felsic plutons of the eastern Coast Plutonic Complex (CPC) that outcrop along the south Klondike Highway in Alaska and British Columbia. Thermal and alternating field step demagnetizing methods yielded stable characteristic remanent magnetization (ChRM) directions for all 29 sites of normal, reversed, and mixed polarity. The ChRM resides in single or pseudosingle domain magnetite and (or) pyrrhotite that is shown to be primary by contact tests with the ∼47 Ma vertical White Pass mafic dikes. Paleopoles from six 56 to 50 Ma (mean 52 ± 2 Ma) Intermontane Belt – Yukon–Tanana terrane (IMB–YTT) units that cannot be explained by tectonic tilt are compared with nine clustered 59 to 46 Ma (mean 52 ± 4 Ma) eastern CPC paleopoles. Both paleopole populations show nonsignificant poleward (northward) translation relative to North America (IMB–YTT, 3.7° ± 5.3°N; CPC, 4.3° ± 6.4°S; overall, 1.2° ± 4.9°S), indicating that northward translation of the accreted terranes ended by ∼58 Ma. Conversely, both populations show clockwise (CW) rotation that is either highly significant or substantial (IMB–YTT, 19.3° ± 10.5 °CW; CPC, 7.1° ± 16.1 °CW; overall 12.8° ± 10.9 °CW). The results are best explained by tectonic rotation from ∼50 to ∼45 Ma of the IMB–YTT as a thin-skin on top of North America during emplacement and co-incident rotation of the massive Eocene plutons of the eastern CPC along the North American margin.

High-precision U–Pb geochronology of the Butedale pluton, British ColumbiaThis article is one of a series of papers published in this Special Issue on the theme of Geochronology in honour of Tom Krogh.

The Butedale pluton, a ca. 100 km long compositionally zoned batholith, is part of the Coast Plutonic Complex that extends the length of the Canadian Pacific coast. Its age and thermal history are relevant to the Butedale pluton’s role as a test case of the Baja–BC hypothesis, as paleomagnetic evidence suggests that it may have formed thousands of kilometres to the south and moved northward along what is now the Coast Shear Zone. High-resolution U–Pb (chemical abrasion – thermal ionization mass spectrometry (CA–TIMS), zircon) analysis of rocks across the width of the Butedale pluton indicates that it is actually made up of at least two distinct magmatic events that formed the West Butedale pluton (ca. 95 Ma) and the East Butedale pluton (ca. 85 Ma). The East Butedale pluton was reheated by the emplacement of a younger adjacent pluton, which may have caused partial Pb loss and resulting excess scatter of 206Pb/238U zircon ages within individual samples. The West Butedale pluton may be the same age as, and part of, the nearby Ecstall pluton, thereby doubling the length of the Ecstall pluton to ca. 200 km. Single-grain, high-precision U–Pb analysis of zircon reveals previously unknown complexity and detail of emplacement and thermal history in the Butedale plutons.

Detrital zircon constraints on terrane ages and affinities and timing of orogenic events in the San Juan Islands and North Cascades, Washington

Detrital zircon geochronology of this report pertains to Cretaceous orogeny in northwest Washington, an event that involved blueschist metamorphism and emplacement of nappes in the San Juan Islands – northwest Cascades thrust system and continental arc magmatism and associated Barrovian metamorphism in the neighboring Coast Plutonic Complex. Structurally low in the thrust system, quartzose gneiss of the Yellow Aster Complex yields an entirely Precambrian suite of detrital zircons, with an age pattern that is similar to that of Ordovician miogeoclinal rocks and the outboard Yukon–Tanana, Yreka, and Shoo Fly terranes elsewhere in the Cordillera. Midway in the nappe pile of the northwest Cascades, sandstone in the Bell Pass Mélange has a zircon age population of 110 Ma, an age that together with the spectrum of exotic materials associated with the sandstone suggests the mélange was a major zone of dislocation during mid-Cretaceous nappe emplacement. At a high level in the thrust system are nappes of the Fidalgo Complex, Lummi Formation, Constitution Formation, and Easton Metamorphic Suite, all with a prominent age peak in the range of 148–155 Ma. These units appear to be mutually related, represent inter-arc marginal basins, and are correlative with terranes in the western Klamath Mountains. The Nooksack Formation, footwall to nappes in the Cascades, has a zircon population at 114 Ma, providing a maximum age of nappe emplacement. The Tonga Formation of the Coast Plutonic Complex bears zircons that indicate a depositional age of <125 Ma, thus yielding a maximum age for the beginning of Barrovian metamorphism and continental arc plutonism in this region.