Holocene Normal Faulting in the Southern Rocky Mountain Trench; Orogenic Collapse Modulated by Glacial Unloading?

Abstract The Southern Rocky Mountain Trench (SRMT) is a conspicuous valley in the eastern Canadian Cordillera. It lies above a sharp change in lithospheric strength and thickness and is occupied by a normal fault thought to have last been active in the Eocene. However, its geomorphic prominence, the occurrence of diffuse regional seismicity including notable historical earthquakes, and a subtle geodetic strain gradient, have hinted that it is still capable of large, surface‐rupturing earthquakes. Using new high‐resolution topographic data, surficial mapping, scarp morphology analysis, and shallow geophysics, we provide evidence of multiple Holocene surface ruptures on the SRMT fault near Columbia Lake, British Columbia. A paraglacial fan surface is vertically offset by ∼3 m, whereas younger channels on the fan surface are only offset by ∼2 m, implying that the fan records cumulative offset from multiple earthquakes. Electrical resistivity tomography data confirm that a normal fault continues beneath the scarp, and a resistivity horizon in the subsurface is offset by ∼9 m. Regional geochronological constraints on paraglacial sedimentation constrain the slip rate to between 0.1 and 1.1 mm/yr. A possible decrease in slip rate from the early Holocene to late Holocene may indicate fault activity was modulated by glacial isostatic adjustment following the last glacial maximum. However, the underlying cause of tectonism may be gravitational collapse of the Cordillera. We suggest the SRMT represents a northern extension of the Intermountain Seismic Belt of the western US, which has hosted damaging normal fault earthquakes in recent decades.