Initial Publication Date: June 14, 2024

Geomorphic Evidence of Quaternary Surface Rupture along the Rocky Mountain – Tintina Trench

Theron Finley, University of Victoria
Edwin Nissen, University of Victoria
John Cassidy, Geological Survey of Canada & University of Victoria
Lucinda Leonard, University of Victoria & Ocean Networks Canada
Israporn Sethanant, University of Victoria
Guy Salomon, University of Victoria
Veronica Prush, New Mexico Institute of Mining and Technology
Brendan Miller, British Columbia Ministry of Forests

Abstract

The Rocky Mountain – Tintina Trench (RMTT) is one of the most conspicuous topographic lineaments on Earth, stretching from Montana through the eastern Canadian Cordillera and into Alaska. It is the locus of major Eocene transtensional faulting and marks a step-change in lithospheric thickness, strength, and thermal properties. There is a moderate level of seismicity in the vicinity of the RMTT, and several historical earthquakes (M 4.8 – 6.1) have caused shaking and minor damage. However, little is known about the paleoseismic record of the RMTT and the potential for larger and more damaging earthquakes along it. We reviewed newly-available high-resolution topographic data (drone and airborne lidar and ArcticDEM) along the Canadian portion of the RMTT and conducted follow up field studies at sites with potentially fault-deformed Quaternary landforms. Here, we present evidence of Quaternary surface rupture at two of those sites. In the southern RMTT near the town of Fairmont Hot Springs, a Holocene alluvial fan surface is disrupted by a 3.6 km long west-facing scarp with an average vertical separation of ~ 4 m. No drainages crossing the scarp are laterally offset. Electrical resistivity tomography reveals a subvertical offset in alluvial and glacial strata beneath the scarp, which we interpret to be due to Holocene normal faulting. These kinematics are at odds with contemporary compressive crustal stress and strain patterns, but consistent with results of thermochronological studies in the region. In northwestern Yukon, near Dawson City, a 120-km-long series of scarps and "mole-tracks" disrupts late Pliocene (~2.7 Ma) glaciofluvial outwash deposits. A short segment of the scarp crosses a younger inset glaciofluvial terrace speculated to be related to the MIS 6 (~200 ka) Reid Glaciation on the basis of soil weathering profiles. Terraces related to the most recent (~22 ka) McConnell Glaciation appear undeformed. These results are the best evidence to date of Quaternary surface-rupturing earthquakes along the RMTT and, in addition to the implications for seismic hazard, significantly change our understanding of the Cenozoic evolution of the Canadian Cordillera.

Session

Neotectonics
earthquake geology