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 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.