THE CASE FOR PROLONGED NEOGENE REGIONAL UPLIFT IN BC'S INTERIOR PLATEAU

The origins and fates of orogenic plateaux, and the development of topography and relief, have become ‘hot-topics' in tectonics in recent years, in particular in the Great Basin and Andean Altiplano regions. Studies in the Canadian Cordillera have tended to focus on Paleogene and Mesozoic orogenesis and Pleistocene isostasy as the important controls on the development of the Coast Belt in particular. However, there appears to be little information available on the Miocene-Pliocene physiography, nor its evolution into the Pleistocene despite knowledge of upland glaciations in British Columbia back to at least ~ 3 Ma, and significant drainage reversals (e.g., the Fraser River) within the last 3 Ma.

We wish to draw attention to the abundance of physiographic evidence for sustained surface uplift in the Intermontane Belt of southern and central British Columbia throughout the Neogene that is preserved in and adjacent to the Chilcotin Group basalts. This is recorded by: (1) extensive plateaux repeatedly incised by steep-sided, valleys and canyons and then infilled; (2) several abandoned drainage systems perched-up on the plateaux; (3) proximal, immature, sedimentary rocks and placer-Au deposits in the Fraser River Basin; and (4) successive, locally-extensive terrestrial disconformities and unconformities within the Chilcotin Group basalts. We propose that the distribution, stratigraphy, and physical volcanology of the Chilcotin Group rocks directly result from the physiography and environment at the time of eruption between ca.18 – 0.01 Ma, and beginning at 3 – 0.01 Ma in particular.

Three provisional conclusions will help guide our future studies. (1) That concepts of an extensive “plateaux-forming” flood basalt province can be discarded; rather the Interior Plateau is a relic from the Eocene-Oligocene as it is elsewhere in southern BC. (2) The Chilcotin Group adequately records the evolving Neogene drainage, and thus can be used as a first-order constraint on the location of the Fraser, Chilcotin, Thompson, and Clearwater Rivers and their drainage systems. (3) Combined thermochronological and paleoaltimetric studies, integrating the stratigraphy, geochronology, and paleo-environmental data within the Chilcotin Group, will record the vertical tectonic history of the Interior Plateaux.