An integrated field, petrographic , structural , and geophysical analysis of the Panther Creek half -graben (PCHG), in east -central Idaho, was undertaken to determine its tectonic origin. Two competing theories as to the origin of the PCHG exist. One suggests that it formed as part of the TransChallis fault system (TCFS) , a system of northeast-trending normal faults, eruptive centers, and graben that formed in Middle Eocene time. The graben of this system are bounded by northeast-striking normal faults, and show evidence of syn-tectonic deposition of basin-filling volcanic strata. The other theory suggests that the PCHG may have evolved as a result of a post-Challis volcanic extension event (Late Eocene to Early Oligocene). Graben of this event are bounded by NNW-striking normal faults, and Challis volcanic strata are uniformly tilted; this evidence suggests post-volcanic basin formation. This study demonstrates that formation of the PCHG was mostly synchronous with volcanism and that there was very little post-volcanic deformation. Other conclusions of this research are 1) Rates of slip on basin-bounding faults were high, and slip was episodic. Slip occurred on both NE and NNWstriking faults, but more occurred on the NE-striking basin-bounding fault. This is consistent with the dominant NE strike of faults within the TCFS. 2) Most of the older volcanic units in the basin dip more steeply to the SE than overlying younger units , this is evidence for synvolcanic deposition . 3) There is significant hanging wall deformation in the form of folds and normal faults, particularly in the northern and southern portions of the basin . 4) Paleocurrents determined from provenance studies and imbricate fabric in exposed conglomerate beds indicate a flow direction largely to the northwest. This paleocurrent flow crosses the strike of the syn-extensional basin-filling units . 5) The composition of the clasts in the post-volcanic basin -fill deposits suggests that the thick volcanic units preserved in the basin never blanketed highlands NE, E, and SE of the basin and that the footwall of the basinbounding fault system was the main source of sediment in the basin. 6) The magnetic and bouguer gravity surveys indicate a sizable intrusion (most likely related to the nearby Casto pluton) beneath the west-central portions of the basin. The intrusion(s) may be responsible for east-plunging folds in the tuff of Castle Rock. 7) Work initiated during this study to determine the 40Ar/39Ar incremental-release mineral ages and the polarity of the tuff of Castle Rock and the tuff of Challis Creek will help determine the relationship between these two units and will precisely date basin development. However, it is clear that the basin was nearly fully formed by the end of Challis volcanism.

The southeastern Salmon basin (SB) of east-central Idaho is a complex east-dipping half graben containing four unconformity-bounded sequences of Tertiary volcanic, alluvial fan, lacustrine, and fluvial deposits. From oldest to youngest these are the Challis volcanic group, sedimentary rocks of Tendoy (new name), sedimentary rocks of Sacajaweja (new name), and Quaternary-Tertiary deposits . The thick sequence of volcanic rocks was deposited in a southeast-trending paleovalley . New mapping, cross-cutting relationships, 40Ar/39Ar age determinations, and angular unconformities show that the SB has experienced at least four episodes of extension, and that it lies in the core of an Eocene to Oligocene rift zone. T he largest episodes of extension in the SB were the second and third episodes of extension. These were responsible for the deposition of the sedimentary rocks of Tendoy and sedimentary rocks of Sacajaweja, respectively. Episode 2 occurred along the west southwest-dipping Agency Yearian fault, and can be attributed to gravitational collapse of the Sevier fold and thrust belt between the late middle Eocene and Oligocene (?). Several southwest-dipping low-angle normal faults were active during the third episode and may have been active into early Miocene time. The third episode of extension reflects continued gravitational collapse. Also active during episode 3 was the Salmon basin detachment fault, which is interpreted as a regional detachment based on its lateral extent and low dip angle (11°). Extensional folds are common in the southeastern Salmon basin and represent at least two generations of folds. Interference between northnortheast and southeast-trending folds locally produced dome and basin features. The folds are typically open to gentle, and have a maximum fold height of 2.2 km. Most of the folds are fault-bend folds, and some are associated with growth strata. The development of the Tendoy anticline and Pattee Creek syncline in the hanging wall of the Agency-Yearian fault produced two depositional basins during the deposition of the sedimentary rocks of Tendoy. Facies patterns and megabreccia deposits in the subbasins indicate that there was considerable topography along the margins of the subbasins .