FAULTS IN INTERIOR BRITISH COLUMBIA EXPOSED IN QUATERNARY OUTCROPS; TECTONIC, LANDSLIDE, OR GLACIGENIC?

During recent hazard studies in British Columbia for cross-province pipelines, streamcut exposures of Quaternary glacial and fluvial deposits were observed to contain faults. Slip senses ranged from reverse to normal to strike-slip. Faults commonly occurred in parallel sets, and some faults appeared to have acted as conduits for injection of liquefied sand and gravel. Most faults and fault sets could not be traced away from the exposure as a surface feature (scarp or lineament). These characteristics suggested that perhaps the faults were not tectonic, but related to local processes at the outcrop scale, such as landsliding. To complicate the situation, central British Columbia and its Rocky Mountain Trench were covered multiple times by the Cordilleran Ice Sheet and last uncovered roughly 15 ka. Thus observed faults could be tectonic, landslide, or glacigenic. The author proposes some rules of thumb for discrimination tectonic, landslide, and glacigenic faults in formerly glaciated terrain. First, date the deformed deposits. Deposits of the latest glaciation (MIS2) should not have been overridden by any subsequent glaciations, so should not contain glacigenic faults such as ice-push reverse faults; whereas deposits from earlier glaciations were presumably overridden by MIS2 glaciers and deformed then.

Tectonic Faults: are coincident with or parallel to mapped bedrock faults; same sense of slip as in latest bedrock faulting; no evidence near exposure of landsliding; should be able to trace fault strands away from exposure as a surface feature (scarp or lineament)

Landslide Faults: look for evidence of headscarp(s) above exposure; thrusts in lower half; if slide has been half-eroded, lateral margins may appear as strike-slip faults

Glacigenic Faults: deformation of unconsolidated deposits by ice-push, reverse faults or ice melting/collapse normal faults; often associated with soft-sediment deformation & liquefaction, due to high pores pressures (as in gravifossum). In the published literature, there is no mention of strike-slip glacigenic faulting.

Hazards due to these three fault types are very different, both for avoidance vs. mitigation, and for the regulatory requirements.