Cenozoic thermal evolution and tectonics of the Coast Mountains of British Columbia: 1. Fission track dating, apparent uplift rates, and patterns of uplift

Abstract

The high relief of the Coast Mountains of British Columbia has been produced in part by rapid late Neogene uplift. The relationship of this uplift to the presently diverse and complex plate tectonic regime off the west coast of British Columbia, however, is not clear‐cut. To study this problem, the areal patterns, rates, and total amounts of Cenozoic uplift from 50° to 55°N have been determined by using fission track dating of zircon and apatite separated from rocks collected at high and low altitudes along several traverses through the mountains. Accepting closure temperatures of 105°C ± 10°C and 175°C ± 20°C for apatite and zircon, respectively, and a likely closure temperature difference between zircon and apatite of 70° ± 15°C, paleogeothermal gradients have been measured at Kemano ( °C/km 35 Ma ago) and near Ocean Falls‐King Island ( °C/km 20 Ma ago). Subcrustal heat flow is suggested to have significantly increased in the late Neogene near King Island. Uplift rates in the axial part of the northern (52°–55°N) Coast Mountains were 0.1–0.2 km/Ma from 25 to 15 Ma ago; this uplift was temporally and tectonically related to part of the subsidence in the adjacent Queen Charlotte basin. Uplift rates increased somewhat in the Late Miocene to about 0.4 km/Ma, probably due to passage of the Anahim “hot spot” beneath the area. Late Miocene‐Recent erosion in the north (52°–55°N) has reduced summit altitudes and been more extensive than farther south. Despite this, relicts of Miocene river valleys and topography are still locally preserved, partly beneath 7–10 Ma basalts. The area of the present southern (50° to 52°N) Coast Mountains was near sea level and experienced very low uplift rates (<0.1 km/Ma) throughout the middle Cenozoic (30–10 Ma), despite its more active volcanism. Late Miocene basaltic lavas that were erupted onto a mature erosion surface near Taseko Lakes were elevated and warped by rapid Pliocene‐Pleistocene uplift (>0.5 km/Ma) of the southern Coast Mountains. The profile of this uplift was broadly plateaulike, and this uplift led to the inversion of the Miocene coastal topography. Middle Cenozoic uplift from 52° to 55°N was probably the result of denudation and diminishing uplift after the terminal Paleocene‐Eocene orogenic episode which grossly thickened the crust. This middle Cenozoic erosion resulted in the restoration of a normal crustal thickness. Late Neogene uplift in the Coast Mountains was the result of thermal expansion in the mantle caused by a mantle hot spot near 53°N and changes in the geometry of the subducted slab farther south.