GEOLOGY OF THE GRANITE FALLS 7.5-MINUTE QUADRANGLE AREA—A RICH HISTORY OF NEOTECONIC BASIN DEVELOPMENT AND INVERSION TO MESOZOIC ACCRETIONARY TECONICS IN THE FOOTHILLS OF SNOHOMISH COUNTY, WASHINGTON

The ENE-trending Explorer Falls basin (EFB) is mapped in the Granite Falls, Lake Chaplain and Lake Roesiger 24k quadrangles. We use composition, sedimentology, geochemistry, stratigraphy, OSL/IRSL and detrital zircon ages to show that nonglacial basin fill is 1) ancient Pilchuck River alluvium derived from the Cascade Range (similar to modern and ancient Skykomish and Snoqualmie River alluvium south of the area), 2) correlative with at least four, early to late Pleistocene nonglacial intervals, and 3) locally inverted from on-going regional north-south compression. The EFB is bounded on the north by the Carpenter Creek fault (CCF). Seismographs deployed in the area detected earthquakes (up to M3.4) along an ENE-trending band that we correlate broadly with the active CCF. Directly south of the CCF and in the earthquake band is the Pilchuck River anticline, interpreted as a mid-to late-Pleistocene growth fold and is one of several active to potentially active basin inversion structures in the EFB.

The NW-trending, dextral to oblique-slip Granite Falls fault zone (GFFZ) forms the eastern Everett basin structural margin in the quadrangle. Extensional “pull apart” structures in the GFFZ and Iron Mtn fault zone were intruded by the syn-tectonic Granite Falls stock (GFS) and related dike complexes. Rhyolites near Granite Falls are extrusive equivalents of the GFS. Regional transtension and magmatism at this time may be related to Farallon slab break-off.

Ultramafic rocks are widespread within and west of the GFFZ and may demark where the Eastern mélange belt thrust over the Western mélange belt (WMB) in the latest Cretaceous to early Tertiary. The Lake Chaplain and Sultan River thrusts divide the WMB into three nappes. The intermediary Lake Chaplain nappe (LCN) is predominantly tectonized gabbroic gneiss and metagabbro. An older-on-younger relationship of detrital zircon maximum depositional ages is consistent with an accretionary thrust stacking model with youngest metasandstone detrital zircon ages of 74 Ma and 166 Ma at low and high nappe levels, respectively. Stratigraphically coherent, low-grade metasedimentary and metavolcanic rocks are common above and below the LCN—and in the eastern part of the quadrangle—indicating that penetrative, meso-scale mélange disruption is not common in these bounding WMB nappes.