Along‐Strike Variation in the Magmatic Tempo of the Coast Mountains Batholith, British Columbia, and Implications for Processes Controlling Episodicity in Arcs

Abstract

The growth of the Coast Mountains batholith has been documented as episodic through time, and it has become a type example of a continental arc system that developed through non‐steady‐state magmatism. The magmatic record, however, is not well known along the length of the arc, hindering evaluation of the processes controlling the tempo and patterns of batholith growth. A new, robust geochronologic database (485 U‐Pb zircon and titanite ages, 120 of which are newly presented herein) covering nearly 1,000 km of arc length reveals significant along‐strike variation in the tempo of batholith emplacement, the timing of arc cessation, and the arc cooling history. Zircon ages range from ~180 to 40 Ma along the length of the arc and overlap with titanite ages, with the exception of parts of the central batholith where Eocene extension and exhumation of lower crustal rocks led to a more complex history. New analysis of zircon ages reveals significant along‐strike differences in the timing of high flux magmatic events. Small‐scale (<150 km) intra‐arc variations in magmatic tempo suggest that small‐scale processes, likely operating within the arc system, appear to have driven the episodic growth of the Coast Mountains batholith. In contrast, rates of Cretaceous‐Paleogene eastward arc migration are consistently ~2.5 km/Myr along the length of the arc. These rates are similar to those documented in North American arc systems, which suggests that arc migration has an external, plate‐scale driver and/or is an intrinsic, self‐modulating feature of most continental arcs.

Subduction Polarity in Ancient Arcs: A Call to Integrate Geology and Geophysics to Decipher the Mesozoic Tectonic History of the Northern Cordillera of North America

“…from ca. 120-80 Ma (Cecil et al, 2018). This rate, age, and direction of arc migration are also shared by the Sierra Nevada and Peninsular Range batholiths, which are interpreted to have faced to the west in nearly all Cordilleran syntheses.…”

U-Pb and Hf Isotopic Analysis of Detrital Zircons From Paleozoic and Cretaceous Strata of Southern Vancouver Island, British Columbia

“…Black arrows show interpreted crustal evolution trajectories assuming present-day 176 Lu/ 177 Hf = 0.0093 (Vervoort and Patchett,1996;Bahlburg et al, 2011). (Gehrels et al, 2009) and Southern Coast Mountain Batholith (Cecil et al, 2018) are represented with dashed lines. Reference lines on the Hf plot are as follows: DM-depleted mantle, calculated using 176 Hf/ 177 Hf0 = 0.283225 and 176 Lu/ 177 Hf0 = 0.038512 (Vervoort and Blichert-Toft,1999); CHUR-chondritic uniform reservoir, calculated using 176 Hf/ 177 Hf = 0.282785 and 176 Lu/ 177 Hf = 0.0336 (Bouvier et al, 2008).…”

Stitch in the ditch: Nutzotin Mountains (Alaska) fluvial strata and a dike record ca. 117–114 Ma accretion of Wrangellia with western North America and initiation of the Totschunda fault

“…In southern Alaska, these igneous products include the Early to Late Jurassic Talkeetna arc (Rioux et al, 2007), the Late Jurassic to Early Cretaceous Chitina arc (Plafker and Berg, 1994), and the Early Cretaceous Chisana arc (Barker et al, 1994). In southeastern Alaska and northern coastal British Columbia, arc plutons within the Insular terranes comprise the western Coast Mountains batholith (Gehrels et al, 2009;Cecil et al, 2011Cecil et al, , 2018. The allochthonous Yakutat terrane is faulted against the outboard edge of the Chugach terrane and has been subducting at a shallow angle beneath the subduction complex and Insular terranes since Neogene time (e.g., Enkelmann et al, 2010;Worthington et al, 2012;Arkle et al, 2013).…”

“…Eastern Gravina belt strata accumulated along the western margin of the Stikine, Yukon-Tanana, and Taku terranes. The history of juxtaposition of western and eastern assemblages is obscured by subsequent plutonism, deformation, and metamorphism within the Coast Mountains orogeny (Gehrels et al, 2009;Cecil et al, 2011Cecil et al, , 2018.…”