NO STRAIN LEFT BEHIND: THE PUGET LOWLAND NEOTECTONIC FAULT NETWORK (Invited Presentation)

The Puget Lowland (PL) is compressed against the Canadian Coast Mountains buttress by the thick, northward-migrating basaltic forearc terrane of Siletzia, causing Quaternary deformation. Our interdisciplinary fault mapping, conducted over the last 12 years, shows the pattern of deformation within the PL is complex, with reverse faults accommodating N-S shortening linked by strike slip transfer zones.

Geologic and geophysical mapping supports a connection between the Rattlesnake Mountain fault zone (RMF) and the Southern Whidbey Island fault zone (SWIF) via several Quaternary-active strands along the western foothills of the Cascade Range. The SWIF-RMF forms a ~19 km wide and 130+ km long dextral zone contributing to northward translation of Siletzia, situated near its eastern margin. Eastward, the zone connects to the Yakima fold and thrust belt by way of a series of faults crossing the Cascade Range. Locally, sinistral fault strands form a system conjugate to the SWIF-RMF zone including the active Cherry Creek fault zone. Northeast of the SWIF in the Monroe area, N-S shortening occurs on reverse faults and an anticline/syncline pair that appear to be active in the Quaternary; they are broadly associated with the Kingston arch but lie east of the SWIF-RMF. In central PL, the south-dipping Seattle reverse fault connects the dextral RMF to the sinistral Saddle Mountain fault, thus forming a transpressional zone that facilitates northward motion of the forearc.

The fault distribution and geometry indicates crustal inheritance controls their development. Active structures seem to reactivate well-oriented older structures, e.g. the Monroe fault zone, which formerly accommodated extension, but now accommodates compression. Predominant fault type and geometry correlates with specific geologic units. Within the zone of the forearc occupied by the Western Mélange Belt (WMB), a distributed network of predominantly strike-slip faults exists (e.g. SWIF-RMF) striking at a lower angle to the maximum compression direction. Within Siletzia, we see a sparser fault network, with fewer strike-slip faults strike at higher angles relative to the maximum compression direction. We speculate that this is due to the contrast in the orientation of inherited structures between the WMB accretionary rocks and Siletzia basalts.