CONNECTING THE YAKIMA FOLD AND THRUST BELT TO ACTIVE FAULTS IN THE PUGET SOUND, WASHINGTON: EVIDENCE FROM HIGH-RESOLUTION AEROMAGNETIC AND REGIONAL GRAVITY ANOMALIES

High-resolution aeromagnetic data collected across the north Cascades and northwestern Yakima fold and thrust belt (YFTB) reveal structures in the YFTB and their possible link to active faults in the Puget Lowland. The survey, conducted in 2008, was flown 250 m above terrain along lines spaced 400 m apart. Upper crustal rocks in the survey region have diverse magnetic properties, ranging from highly magnetic rocks of the Miocene Columbia River Basalt Group (CRBG), with both normal and reverse polarities, to nonmagnetic Tertiary sedimentary rocks underlying the CRBG. High-gradient magnetic lineaments correspond with mapped faults and folds of the CRBG and indicate where these structures extend beyond surface exposures. Regional gravity anomalies, on the other hand, emphasize the thickness of sub-CRBG Tertiary sediments linked to topography on underlying pre-Tertiary basement (Saltus, 1993), thereby potentially illuminating faults that extend to basement depth. A two-dimensional model of the northwest-striking Umtanum Ridge fault zone, based on magnetic and gravity anomalies, geologic mapping, and boreholes that penetrate sub-CRBG strata, consists of three thrust faults and associated folds that deform the CRBG. Umtanum Ridge itself is modeled as a transpressional structure within the Cle Elum–Wallula deformation zone (CLEW), uplifted 1 to 2 km along northwest-striking thrust faults with opposing dip. Subtle gravity anomalies suggest that one or more of these faults offsets pre-Tertiary basement.

Structures along Umtanum Ridge can be traced westward into older rocks of the north Cascades and appear linked to active faults of the Puget Lowland and to the White River–Naches River fault zone. The Umtanum Ridge fault zone continues NW through the Cascade Range where strain is transferred to the southern Whidbey Island fault, which truncates the Seattle fault about 35 km east of Seattle. The distribution of earthquakes (M_W ≤ 5.3) during the past 35 years suggests that this throughgoing structure is seismically active east of Seattle, and GPS indicates that north-south compression increases westward along this zone from about 2 mm/yr in the Cascadia backarc to about 4.5 mm/yr in the Puget Lowland.