Strain partitioning in an obliquely convergent orogen, plutonism, and synorogenic collapse: Coast Mountains Batholith, British Columbia, Canada

Abstract

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.

Geochemical evidence of a near-surface history for source rocks of the central Coast Mountains Batholith, British Columbia

“…The western Coast Mountains, which includes the Alexander and Wrangellia terranes, is interpreted to have been juxtaposed with the eastern Coast Mountains through a combination of sinistral translation and contraction during the Early to middle Cretaceous Monger et al 1994;Gehrels et al in press). Throughout the Coast Mountains are multiple individual faults/ductile shear zones, forming a west-vergent thrust belt, that were active from the late Early Cretaceous to the early Tertiary McClelland et al 1992;Journeay and Friedman 1993;Monger et al 1994;Andronicos et al 1999Andronicos et al , 2003Rusmore et al 2000;Stowell and Crawford 2000). The magnitude of shortening accommodated by these west-vergent faults is poorly constrained; however, exposed footwall blocks do yield peak metamorphic pressures in excess of 7 kbar (Rusmore et al 2005).…”

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

“…More recent geologic and/or geochronologic studies in the region were conducted by Douglas (1986), Heah (1990Heah ( , 1991, Ingram and Hutton (1994), Klepeis et al (1998), Crawford et al (1987Crawford et al ( , 1999Crawford et al ( , 2000, Saleeby (2000), and Gehrels (2001) in southernmost southeast Alaska and northernmost coastal British Columbia, by Armstrong and Runkle (1979), Harrison et al (1979), Parrish (1983), van der Heyden (1989van der Heyden ( , 1992, Gareau (1989Gareau ( , 1991aGareau ( , 1991bGareau ( , 1991c, Gehrels et al (1991), Gareau et al (1997), Chardon et al (1999), Andronicos et al (1999Andronicos et al ( , 2003, Klepeis and Crawford (1999), Gareau and Woodsworth (2000), Hollister and Andronicos (2000, and references therein), Rusmore et al (2000Rusmore et al ( , 2001Rusmore et al ( , 2005, Butler et al (2001aButler et al ( , 2002Butler et al ( , 2006, Friedman et al (2001), Chardon (2003), Davidson et al (2003), and Hollister et al (2004) in central portions of the study area, and by Rusmor...…”

Constraints on the depth of generation and emplacement of a magmatic epidote‐bearing quartz diorite pluton in the Coast Plutonic Complex, British Columbia

“…6,[480][481][482][483][484][485][486][487][488] . (Hollister and Crawford, 1986;Crawford et al, 1987;Andronicos et al, 1999) and (3) crustal extension and exhumation during detachment faulting between 60 and 48 Ma (Hollister and Andronicos, 2000;Andronicos et al, 2003). Our study suggests that the CPC represents a stable, long-lived arc or arcs in which intermittent magmatism occurred between $188 and 90 Ma, including intrusion of magmatic epidote-bearing plutons that cooled nearly isobarically at depth.…”

“…Two composite terranes, the Intermontane and Insular superterranes (Monger et al, 1982), are separated by the Coast shear zone (CSZ), a near vertical, crustal scale shear zone that parallels much of the length of the Canadian Cordillera and records a polyphase deformation history that includes dextral transpression (85-57 Ma; Andronicos et al, 1999Andronicos et al, , 2003 and transtension with northeast-side-down normal motion (57-48 Ma; Klepeis et al, 1998;Andronicos et al, 1999) (Fig. 1).…”