Measurement of crustal flexure in the Lake Hills, South Central Idaho and Timing of Eastern Snake River Plain Subsidence

Late Miocene rhyolite along the north flank of the Eastern Snake River Plain (ESRP) near Carey, Idaho, was studied to investigate the flexure and subsidence history of the ESRP. The rhyolite has been divided into three formations that include seven individual ignimbrite flows. A petrology study of 26 vitrophyre samples, from the lower 4 flows, revealed 3 distinct formations based on percentage of small, large and skeletal feldspars, degree of welding and percentage of glomerocysts. Formations Tiv O (oldest) and Tiv Y (youngest) are indistinguishable in hand sample and in outcrop; however, the middle formation, Tiv M, harbors a distinct phenocryst-poor and thin vitrophyre. New 40Ar/39Ar analysis yields ages of 9.21±0.18 Ma and 9.16±0.20 Ma for Tiv O, 8.39±0.54 Ma for Tiv M and 8.76±0.38 Ma for Tiv Y. Field mapping and measurement of ignimbrite compaction foliations delineates three structural domains. Together the three domains exhibit a map-scale east-trending flexural antiform. Domain 1 extends 0-4 km north of the ESRP. Stratigraphically up section, southerly dips have an average of Tiv O 10°, Tiv M 7°, Tiv Y 7° and for Quaternary basalt 5°. With a slightly undulating topography, Domain 2 stretches 4-6 km north of the ESRP and displays dips of 10-21° to the north and south. All unit groups are present within Domain 2; however, in numerous locals Tiv M is directly underlain by Challis volcanics (Tcv) and where Tiv O is present, the unit is thinner. Domain 3 extends 6-12km north of the ESRP and has predominantly north dipping foliations between 10-21°. As in Domain 2, Tiv O is thinner or not present throughout domain 3. We interpret the formation of the antiform as a crustal flexure response to the subsidence of the ESRP. The northward thinning of Tiv O, suggests a topographic high in the previous paleotopography within Domain 2 and the initiation of the subsidence of the ESRP prior to rhyolite deposition. Two distinct angular unconformities between Tiv O-Tiv Y and Tiv Y-Qb, further indicate that subsidence continued during and after the emplacement of the rhyolite. Previous studies of crustal flexure along the northern edge of the ESRP have been conducted at Howe Point (100 km NE of the Lake Hills) and Lidy Hot Springs (130 km NE of Lake Hills). Together with the new data from the Lake Hills presented within, over 130 km of crustal flexure has been identified and analyzed along the northern boundary of the ESRP. The Lake Hills experienced flexure before, during and after ignimbrite deposition. Two episodes of flexure occurred at Howe Point. Major flexure (25°) occurred from 16-10 Ma while minor flexure commenced after 6.0 Ma. Lidy Hot springs experienced major flexure before 7-10 Ma and minor flexure after 6.0 Ma. Major flexure and ESRP subsidence has been was previously proposed to signify an isostatic crustal response to the emplacement of plutonic loads below the ESRP. However, initiation of crustal flexure preceded the emplacement of Yellowstone hotspot ignimbrites in all three locations. Only minor flexure has been identified in all three locations during or after ignimbrite deposition, suggesting plutonic loading had already occurred prior to ignimbrite deposition.