ONION-LIKE MAP PATTERN AND BASEMENT-INVOLVED THRUSTING IN THE BOEHLS BUTTE-MARBLE CREEK AREA OF NORTHERN IDAHO

Recent geologic mapping and U-Pb zircon geochronology support a revised structural interpretation of the Boehls Butte-Marble Creek area in the western part of the Clearwater complex of northern Idaho. This area exposes the following structural stack, from lowest to highest: 1) possibly autochthonous Goat Mountain plate containing upper amphibolite facies aluminous schist, 1.79 Ga anorthosite, 1.59 Ga amphibolite, and minor undated orthogneiss; 2) allochthonous Cornwall Creek plate containing 1.86 Ga basement orthogneiss, nonconformably overlying 1.5(?) Ga Neihart/Gold Cup Quartzite, and disconformably(?) overlying lower amphibolite facies semipelitic 1.47 Ga Prichard Formation of the Belt Supergroup and its 1.47-1.45 Ga mafic intrusions (Moyie sills); 3) Grandmother Mountain plate consisting of 1.86 Ga orthogneiss and middle amphibolite facies coarse-grained garnetiferous schist and quartzite; and 4) Lookout Mountain plate of greenschist facies muscovite-rich schist, quartzite, and calc-silicate rocks of the 1.45 Ga Ravalli and Piegan Groups. These four plates are separated from lowest to highest by the Jug Rock shear zone, Cluggs Jumpoff fault, and Widow Mountain fault. A structural culmination centered on the high-grade Goat Mountain plate forms a map pattern resembling a sliced onion, with lower grade rocks of the Cornwall Creek plate wrapping entirely around the culmination and in turn being partly surrounded by higher grade rocks of the Grandmother Mountain plate. Tops-to-the-east extensional faulting along the Jug Rock shear zone has likely omitted part of the stratigraphic section, whereas the overlying Cluggs Jumpoff fault is interpreted as a thrust. The Widow Mountain fault is likely a normal fault, but may have had an earlier thrust history. It is not clear how much of the extensive WNW-ESE stretching lineations and (sheath?) fold axes within the plates formed during thrust stacking versus Eocene extension. It is apparent, however, that basement rocks are involved in thrusting because in places they are thrust over lower grade Belt Supergroup rocks. This portion of the Clearwater complex thus provides a window into the hinterland of the Sevier fold and thrust belt and illuminates the structural complexity likely present at depth elsewhere in the orogen.