The Newport fault: Eocene listric normal faulting, mylonitization, and crustal extension in northeast Washington and northwest Idaho
The Newport fault is a spoon-shaped, shallowly dipping fault zone, across which a Proterozoic to Tertiary sedimentary suprastructure is juxtaposed above an infrastructure of basement gneiss and granitic batholiths as a result of Eocene normal faulting and crustal attenuation. Chloritic microbreccia occurs at the top of the Newport fault zone, below which a zone of mylonitization as much as 500 m thick is developed in footwall rocks, including the Eocene Silver Point Quartz Monzonite. Detailed kinematic analysis of fabric and structures in the mylonite and microbreccia establishes that displacement was normal in sense on both sides of the U-shaped fault trace. The direction of extension was 74°-254°, as shown by consistently oriented mylonitic lineation. Crustal attenuation across the Newport and adjacent Purcell Trench faults may have reached as much as 68 km (120%), and, in any case, it exceeded 35 km (40%). During faulting, the footwall moved up and out to the east and west relative to the hanging wall, forming two flanking footwall culminations or crustal-scale boudins. Tectonic denudation of the footwall infrastructure is shown by Eocene K-Ar cooling ages for granitic rocks within it. Chrontours that increase in age outward from the Newport fault record abrupt cooling of the infrastructure as it was drawn away from the hanging wall. The Silver Point Quartz Monzonite was emplaced into the dilatant zone between the footwall infrastructural culminations. During displacement, hanging-wall strata were rotated down the listric-fault surface, the footwall rotated upward, and the fault surface flattened, probably as a consequence of isostatic adjustment to mass redistribution caused by normal faulting. A roll-over anticline developed in the hanging wall as it moved into the zone of attenuation between the footwall culminations. Syntectonic Eocene extrusive rocks and coarse clastic deposits accumulated in a growth fault basin on the west flank of the hanging-wall anticline. Crustal extension and normal displacement on the Newport fault are compatible with a regional Eocene strain regime that crossed northern Idaho, Washington, and southern British Columbia and produced dextral transcurrent faulting, core-complex development, and clockwise rotation of crustal blocks throughout that area.