Ph.D. Thesis – E. Bordet
Eocene volcanic response to the tectonic evolution of the Canadian Cordillera – University of British Columbia, 2014 – Available digitally through UBC (pdf, 79 MB)
Abstract: The Ootsa Lake Group (OLG) represents a voluminous episode of Eocene volcanism across the Interior plateau of British Columbia (BC), in the Canadian Cordillera. Remarkable aspects of the rocks (lithology, texture, volume, extent, and geochemistry) suggest that even though they formed along an active continental margin, the tectonic setting was different from a classic arc. The OLG was defined by field mapping, U-Pb and ⁴⁰Ar/³⁹Ar geochronology, major and trace elements geochemistry, and three-dimensional modelling of the thickness and structure. A new tectonic model for the evolution of the Canadian Cordillera in the Paleogene is proposed using this comprehensive dataset. The OLG stratigraphy comprises a thick sequence of rhyolite and dacite lava, locally capped by andesite. Onset, duration and termination of volcanism are equivalent across the Interior plateau, and are constrained between 54.7 and 46.6 Ma by new U-Pb and ⁴⁰Ar/³⁹Ar geochronology. OLG lavas yield a “volcanic arc” signature (diagnostic high-K calc-alkaline trend, and trace and rare earth elements patterns), suggesting supra-subduction zone contributions from a hydrated mantle wedge. However, a similar signature may be inherited from partial melting of crustal reservoirs composed of older accreted volcanic arc crust. This is supported by Sr isotope data indicating variable crustal contributions to melts across BC. OLG intermediate rocks were likely derived from mantle melting, but dominantly silicic compositions support partial melting of the crust as a dominant magma producing mechanism. Eocene volcanic rocks cover at least 65,000 km² of BC, but their original extent may have been almost continuous from southwestern Yukon to Idaho. Coeval volcanism and extensional deformation contributed to the accumulation and preservation of volcanic products in extensional basins, up to 4000 m thick in some locations. With such dimensions, the OLG may have attained the status of a Silicic Large Igneous Province prior to erosion. The cause of OLG volcanism and coeval extension is attributed to the sudden ingress of hot sublithospheric mantle within a previously metasomatized mantle region, following cessation of subduction and a slab break. The resulting thermal anomaly progressed across a “slab gap” beneath BC, leading to mantle and lower crustal melting, crustal anatexis and magmatism.
Geoscience BC Report 2016-12
Distribution and Nature of the Eocene Ootsa Lake Group in the Chilcotin Plateau, parts of Quesnel and Anahim Lake map areas (NTS 093B & 093C), central British Columbia
by Esther Bordet and Craig J.R. Hart, MDRUGeoscience BC Report 2016-12 contains a new geological map of a portion of the Chilcotin and Nechako Plateau. The map highlights previously unmapped rock outcrops and will support mineral exploration in aregion which is known for poor rock exposure. The 1:100 000 scale geology map is in pdf format, and supported with GIS files, a field and analytical database and a brief report.
Geoscience BC Report 2011-13
Preliminary Lithological and Structural Framework of Eocene Volcanic Rocks in the Nechako Region, Central British Columbia
by E. Bordet, C. Hart and D. MitchinsonThis report presents the preliminary interpretations of an ongoing PhD thesis project by Esther Bordet. The report documents new mapping and proposes an improved stratigraphic and structural model for the Eocene period in the Nechako Basin. Characterization of the nature, thickness and structural framework of Eocene volcanic rocks in the Nechako region provides new insights into the area’s Early Cenozoic history, contributes to improved interpretations and adds value to existing geophysical, particularly seismic and magnetotelluric, data sets.
The proposed GIS thickness model combined with interpreted structural and stratigraphic lineaments can be compared with magnetotelluric and seismic 2D profiles to define the structure of the underlying basin and interpret the abrupt changes in the thickness of the overlying Paleocene and Eocene packages. In addition, physical property measurements of mapped lithologies help constrain the geophysical responses of these rocks.
The new stratigraphic and structural model proposed for the Eocene volcanic sequence will provide a more robust framework for future mineral and oil & gas exploration in the Nechako region.