Date of Award

2018

Document Type

Open Access Thesis

Department

Earth and Ocean Sciences

Sub-Department

College of Arts and Sciences

First Advisor

Andrew Leier,

Abstract

Eocene sedimentary strata exposed in the interior of the southern Canadian Cordillera (SCC) in British Columbia (BC) and northernmost Washington (WA) record a poorly understood history of extension-related deformation in the hinterland of the orogen. Today these strata are exposed in isolated, distinct outcrop belts, although the nature of the original basin(s) is unknown. We examined 650 m of Eocene strata, analyzed 2,995 detrital zircons for uranium-lead (U-Pb) ages, and measured 67 detrital zircons for Hf isotope systematics in an effort to better understand the physiography of the SCC during this time period. Eocene strata consist of clast- and matrix-supported conglomerates, very fine- to very coarse-grained sandstones, mudstones, and coal that were deposited in fluvial, alluvial fan, lacustrine, and paludal environments. Sandstone samples from across the region contain large populations of detrital zircons with Eocene (ca. 51 Ma) and/or Jurassic (ca. 160) U-Pb ages, which are interpreted to have been derived from the erosion of the Eocene Challis-Kamloops volcanics, and Mesozoic-age batholiths, respectively. Maximum depositional ages (MDA) of the Eocene strata are relatively uniform throughout the region, with most MDA between 47-50 Ma. ƐHf values of ~50 Ma detrital zircons extracted from samples collected in 3 locations (Merritt, BC, Kelowna, BC, and Republic, WA) across the SCC vary between -16 to +14. Detrital zircons from strata in Republic, WA, in the east of the study area, have primarily negative ƐHf values, with one positive value (-16 to +7), indicating derivation from relatively evolved sources. Detrital zircons from strata in Merritt, BC, in the west of the study area, have positive ƐHf values, with one negative value (-2 to +13), indicating relatively juvenile sources. Detrital zircons from Kelowna, BC, in the central part of the study area have bimodal ƐHf values, with both positive and negative populations (-10 to +12). ƐHf values correspond to the depositional location relative to the strontium (Sr) 0.706 isopleth. The localized changes in sedimentary facies, the variability in ƐHf values of 50 Ma detrital zircons, and the local variations in detrital zircon U-Pb ages indicate Eocene sedimentary strata in the hinterland of the SCC were deposited in multiple, isolated basins separated by paleotopographic highs, and not in a single continuous hinterland basin. The MDA data and existing constraints suggest these basins formed across the SCC at approximately the same time, although the basins to the east formed in traditional grabens, those in the central portion formed in a supradetachment basin, and those in the west are associated with strike-slip faulting. The regional transtensional stress field is attributed to the subduction of an oceanic spreading center beneath the SCC during the Eocene, which resulted in oblique subduction along this portion of the North American margin.

Rights

© 2018, Erica May Rubino

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