Two‐stage exhumation of midcrustal arc rocks, Coast Mountains, British Columbia

Abstract

New geologic data from the Central Gneiss Complex along Douglas Channel help delineate the burial and exhumation of the Late Cretaceous to Eocene Coast Mountains magmatic arc. Arc plutonism was on going between ∼90 and 60 Ma, replaced by formation of dikes to 52 Ma. Supracrustal rocks were buried to midcrustal levels by 90 Ma, and garnet and kyanite grew from ∼90 through 70 Ma. Subsequent exhumation was nearly isothermal and took place in two stages. The first occurred from ∼70 to 59 Ma as the arc contracted obliquely and sillimanite replaced kyanite. Exhumation was slow (∼0.5 mm/yr), probably accomplished by erosion aided by coaxial crustal thinning. Exhumation rates about doubled in the second stage of exhumation, after 59 Ma and before ∼52 Ma. Penetrative deformation ended prior to intrusion of the Quottoon pluton at 59 Ma and ∼6.5 kbar. Production of cordierite rims on garnet at 4.5 kbar and ∼700°C signaled the end of near‐isothermal decompression prior to 52 Ma. Rapid cooling (100°C/106 years) of the Central Gneiss Complex followed cordierite growth; temperatures dropped to ∼250° by ∼48 Ma. The increase in exhumation rate and the subsequent rapid cooling are attributed to excision of >6 km of crust on a detachment system on the northeastern side of the Central Gneiss Complex. Comparison to other parts of the Coast Mountains arc, the Sierra Nevada, and Fiordland, New Zealand, shows that the amount and tempo of exhumation vary greatly within and between arcs, suggesting that the processes accommodating exhumation vary significantly.

Assembly of the Lhasa and Qiangtang terranes in central Tibet by divergent double subduction

“…This means that such south-directed thrusting may have been strengthened by the northward subduction of the Neo-Tethyan Ocean lithosphere beneath the southern Lhasa subterrane and thus cannot be considered as a strong argument for the northward subduction of the Tethyan Bangong Ocean lithosphere beneath the Qiangtang Terrane.The absence of the Early Cretaceous high-grade metamorphic rocks along the Bangong suture zone may be attributed to the low degrees of exhumation, analogous to absence of such rocks in parts of the Coast Mountains arc in western North America(Rusmore et al, 2005) and/or to the presence of a sedimentary cover as interpreted for the lack of Jurassic granitoids in places in the western Qiangtang subterrane(Guynn et al, 2006). However, these two possibilities are less likely because no Early Cretaceous high-grade metamorphic rocks have been documented along the entire Bangong suture zone despite the fact that (1) the sedimentary cover for the western Qiangtang subterrane is only ca.…”

Logan Medallist 1. Seeking the Suture: The Coast-Cascade Conundrum

“…Combined, the structures record transpression with a dominantly sinistral component into midCretaceous time and a dextral one after then. The last stage of CCO evolution involved transtension, recorded by Eocene (~55-45 Ma) normal faults in the eastern Coast Mountains (and widespread to the east across southern British Columbia) that are in part coeval with movement on major dextral strike-slip faults (Coleman and Parrish 1991;Rusmore et al 2005).…”

Thermal history of the Ecstall pluton from 40Ar/39Ar geochronology and thermal modeling

“…Agreement between the modeled and observed trends in hornblende ages might be improved if the duration of reheating is longer. Rusmore et al (2005) showed that magmatism in the Central gneiss complex, south of our study area and just east of the Quottoon plutonic complex, was nearly continuous from 90 to 67 Ma, followed by rapid cooling of the entire complex at $52 Ma. Closest to our study area, the Quottoon plutonic complex gives a U-Pb zircon age of 58.6 ± 0.8 Ma (Gehrels et al, 1991).…”

“…Crawford et al (1999) conclude that the Quottoon plutonic complex was emplaced as a series of individual intrusions which would increase the duration of reheating relative to a single intrusion. Also, the timing of metamorphism in the Central Gneiss Complex indicates relatively high temperatures until $52 Ma (Rusmore et al, 2005). An increase in the duration of reheating would increase maximum temperatures reached near the thermal boundary, and increase the distance at which hornblende 40 Ar/ 39 Ar ages are affected.…”