(U-Th)/He Dating in the Coast Mountains, British Columbia

* Farley, K A (1), Rusmore, M E (2), Bogue, S W (2)

(1) California Institute of Technology, MS 170-25, Pasadena, CA 91125 United States

(2) Dept of Geology, Occidental College, Los Angeles, CA 90041 United States

Miocene basin formation, volcanism, and crustal warping have been proposed as linked expressions of significant crustal extension in western British Columbia. Integrated structural studies and (U-Th)/He dating were undertaken to test this hypothesis, and evaluate the exhumation history of the Coast Mountains south of 54$\deg$N. 60 (U-Th)/He ages were obtained on apatites fromDouglas Channel, a range-traversing fiord in the central Coast Mountains. Apatites from the Ecstall and Quottoon plutons and a few metamorphic units give extremely reproducible He ages that range from 18 to 2.5 Ma. Three vertical transects spaced across the orogen yield highly linear age/elevation relationships, implying a slow exhumation rate of 0.2 mm/yr between 10 and 4 Ma. At the highest elevations of the easternmost transect (1500-2000 m) the He age pattern defines an apparent partial retention zone, suggestingthis period of exhumation began after relative quiescence from >19 to 10 Ma. A rate of 0.2 mm/yr is insufficient to exhume rocks from the depth of the closure isotherm to the surface in the last 4 Ma, implying at least a 2x increase in the exhumation rate since 4 Ma. (U-Th)/He ages were also obtained on a 75 km longorogen-perpendicular traverse of the range at sea level. Ages decrease smoothly from $\sim$10 Ma at the seaward end of the traverse near Grenville Channel to 2.5 Ma near the range crest at the N. end of Hawkesbury Island, then increase to $\sim$6 Ma at Kitimat. Brittle faults are widespread across this transect, but only significantly disrupt the pattern at its southern end. The He ages show that significant normal faulting did notaffect the range after 10 Ma, and that regional normal faulting would mostly likely be older than $\sim$19 Ma.Thus, most exhumation of Paleocene metamorphic rocks in the range took place between ca. 60 and 20 Ma. The data also indicate that development of the present mountain range occurred after about 4Ma. Beyond their geologic significance, these data provide compelling evidence that a) internallyconsistent He ages can be obtained on large sample suites from varying plutonic lithologies and b) variationin the He closure temperature must be small ($<$$\pm$3$\deg$C).

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