Piecing Together Western North America

Todd LaMaskin is investigating Mesozoic rocks of eastern Oregon and western Idaho to answer some long-standing questions about the geologic history of the Pacific Northwest. Two-hundred million years ago, the western edge of North America was located in western Idaho and what we think of as “Oregon” was a deep ocean with huge volcanic islands rising from the depths. Ongoing plate-tectonic processes resulted in the addition of these island arcs to the crust of North America via a process called accretion. Rocks that are now exposed in the Klamath Mountains, Blue Mountains, and in Hell’s Canyon, represent Oregon’s geologic history long before the more familiar, modern landscape.

Despite a rich history of investigations in the Blue Mountains, fundamental aspects of the regional geology remain unclear. For example, we do not know if there was a single island arc that lay well offshore of North America, or if the rocks of the Blue Mountains represent multiple island arcs and we do not know when these island arcs were accreted to the crust of the North America. These ambiguities introduce significant uncertainties to tectonic and geochemical models of how the crust of North America has grown over time and hamper our ability to accurately reconstruct the evolution of western North America.

In collaboration with Becky Dorsey (University of Oregon) and Jeff Vervoort (Washington State University), Todd is using trace-element, samarium-neodymium isotopic, and detrital-zircon provenance analysis of mudrocks and sandstones to characterize the tectonic settings of sediment source areas and to constrain the timing of terrane accretion in the region. Flat REE patterns (see figure above), low incompatible-element concentrations, positive ε_Nd values and a lack of Precambrian detrital-zircon grains indicates that during Middle to Late Triassic time, the Wallowa terrane was an intra-oceanic arc system. Conversely, steeper REE patterns, higher incompatible element concentrations, more negative ε_Nd values and ubiquitous Precambrian detrital-zircon grains indicate a continent-fringing setting for the Triassic Olds Ferry arc.

Middle to Late Jurassic deep-marine shales overlying both the Wallowa and Olds Ferry arcs have steepened REE patterns, higher incompatible-element concentrations, more negative ε_Nd values and Precambrian detrital-zircon grains, indicating that following Triassic amalgmation, the entire BMP region received sediment input from continental sources. These results suggest that BMP accretion likely initiated in the Middle Jurassic, earlier than is generally agreed upon (see figure to the right). Integration of these new data with existing information from California, Nevada, and southern British Columbia is generating new insights into the history of Mesozoic terrane accretion and related basin evolution in western North America.