EVIDENCE FOR AN ACTIVE GALES CREEK FAULT - A DYNAMIC DUO ACTIVE DEFORMATION CONFIRMATION

The Gales Creek fault (GCF) is recognized as a major, potentially active structure ~35 km west of Portland, Oregon. We used lidar imagery and field mapping along ~20 km of the GCF to map its surficial expression and locate paleoseismic trench sites. The geomorphic expression of the GCF is clearly visible in lidar imagery. Geomorphic indicators of active strike-slip faulting observed include deflected streams, sidehill benches, sag ponds, and shutter ridges. Strong lineaments cross hillslopes, saddles, and landslides. Faint, but detectable, lineaments and subtle topographic scarps cross some alluvium-filled valleys. Seven trenches were excavated across the GCF to assess recency and recurrence of surface rupturing earthquakes along ~20 km of the fault system. Preliminary age estimates based on IRSL analyses of feldspar and ¹⁴C ages demonstrate the GCF is an active fault and has experienced multiple surface-rupturing earthquakes in the Holocene and latest Pleistocene.

Exposures within 5 trenches reveal bedrock overlain by hillslope colluvium and/or loess deposits with moderate to well-developed soils and IRSL ages ranging from ~200 ka to ~20 ka. Overlying are deposits with weakly developed soils that range in age from ~13 ka to ~2 ka and include loess, slope colluvium, loess-draped scarps, scarp generated colluvium, and pond deposits. The earthquake record varies between trenches and estimates of recurrence and evaluation of coincidence of ruptures along the fault await additional age analyses. In some trenches there appears to have been at least four surface ruptures in the last ~12 ky. Two trenches were excavated across a subtle, single event scarp in Scoggins Valley and exposed ~2 m of fluvial sediments representing 4 or more floods deposited since ~2.5 cal yr B.P. Flood deposits as young as 640 cal. yr B.P. are faulted and represent the most recent event in the central part of the fault zone. Faulting may displace deposits as young as 250 cal. yr B.P., though we await additional analyses to confirm that age estimate.