Age of emplacement of the Okanogan gneiss dome, north-central Washington

Kettle dome and related structures of northeastern Washington

“…Fox and others (1976) proposed that Snook's name of Tonasket Gneiss be applied to all such rocks (Fig. 3).…”

“…On the southeastern margin of the dome, upper Paleozoic hornfelsic phyllite occurs adjacent to an orthogneiss that is similar to the interior porphyritic granodioritic phase of the Tonasket Gneiss. Fox and others (1976) reported U-Pb ages of 87 and 100 m.y. and a Th-Pb age of 94 m.y.…”

Geology of the Elliston region, Powell and Lewis and Clark counties, Montana

“…Interpretations developed from the study area and from adjoining areas challenge the popular concept that places faults along most mountain fronts (Pardee, 1950;Reynolds, 1979). Documentation of Late Cretaceous to middle Eocene tensional tectonics in westcentral Montana fits the history of slip on the Lewis and Clark line (Wallace and others, 1990) and fits the tensional pre-middle Eocene and middle Eocene tectonic regime that has been well documented west of the study area in Idaho and Washington (Fox and Rinehart, 1988).…”

“…not occur in the study area and most likely does not exist at all, and (2) much of the extensional slip attributed to the Basin-andRange tectonic event in the Elliston area during Oligocene and Miocene time (Pardee, 1950;Reynolds, 1979;McMurtrey and others, 1965;Konizeski, 1965;and Csejtey, 1963) is probably Late Cretaceous to pre-middle Eocene in age. This tensional tectonic event could be related to Late Cretaceous slip on faults of the Lewis and Clark line (Wallace and others, 1990) or to an Eocene tensional tectonic event described in northern Washington and Idaho (Fox and Rinehart, 1988;Rhodes and others, 1989). The ancestry of the modem large-scale topographic features in this region dates from this pre-Basin-andRange slip on tensional faults.…”

“…Recognition of Late Cretaceous tensional tectonics and a pre-middle Eocene tensional tectonic regime in and adjacent to the study area carries important implications that extend beyond the Elliston area: (1) The early to middle Eocene and pre-middle Eocene tensional tectonic regime in northern Washington and Idaho described by Fox and Rinehart (1988), Fox and Wilson (1989), Rhodes and others (1989), and Miller and Engels (1975) probably affected west-central Montana, and this early event of tensional tectonics could have shaped much of the present topography; and (2) normal faults, previously considered by many geologists to be of Oligocene and younger Basinand-Range origin, may have numerous tectonic overprints that include Late Cretaceous and pre-middle Eocene periods of slip, and estimates of slip amounts attributed to Basin-and-Range faulting and estimates of the percentage of extension must account for pre-Oligocene normal slip.…”