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Abstract.The

New I4C ages date the eruptions that produced the White River and Bridge River tephras, two important Holocene marker beds in western Canada. The 14C ages were obtained on trees in growth position buried in coarse tephra and a pyroclastic flow near the source vents. The mean calendric age of the White River eruption, based on four I4C ages, is 1147 cal years BP (calibrated years, approximately equivalent to calendric years) or AD 803 (the 2a age range, obtained from the two most precise 14C ages, is 1014-1256 cal years BP or AD 694-936). The mean age of the Bridge River eruption, determined both from (i) the single most precise outer-ring I4c age and (ii) the weighted mean of six outer-ring I 14C ages is 2360 cal years BP or 411 BC (20 age range = 2349-2704 cal years BP or 755-400 BC).

We describe the crustal structure of the Coast Mountains batholith between 54° and 55°N, within the Canadian Cordillera, with emphasis on emplacement of the 7 km thick Kasiks sill complex (KSC). Kinematic patterns that developed during emplacement of the KSC are the result of interactions between magma transport, magma accumulation and regional deformation. The sills were emplaced during NW directed normal shearing and flattening of country rocks that host the KSC. A ∼2 km thick shallowly NE dipping mylonite zone cuts the eastern side of the KSC. Kinematic indicators within the mylonite zone record top to the east normal displacements. Structural analysis shows that mylonite formation occurred during subvertical shortening and east‐northeast, subhorizontal extension. U/Pb zircon age dates show that ENE directed normal shearing along the eastern side of the KSC and WNW directed normal shearing within the KSC occurred contemporaneously between ∼54 and 51 Ma, indicating strong strain partitioning between the mylonite and the KSC. This pattern of strain partitioning is interpreted to have been driven by return flow of melt‐laden crust in response to tectonic denudation of the upper crust. Seismic profiling shows that many of these structures extend to mid and lower crustal depths. Comparison of our results with other regions within the Canadian Cordillera indicates that orogen‐scale right‐lateral strike‐slip faults deformed synchronously with wide spread magmatism and formation of extensional gneiss domes. Thus the crustal structure of the Coast Mountains batholith was the result of early Tertiary batholith construction during dextral oblique convergence and synorogenic collapse.

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.