Rapid extension in an Eocene volcanic arc: Structure and paleogeography of an intra-arc half graben in central Idaho Susanne U. Janecke Department of Geology, Utah State University, Logan, Utah 84322-4505 Brian F. Hammond } Lawrence W. Snee U.S. Geological Survey, M.S. 963, Box 25046, Federal Center, Denver, Colorado 80225 Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, John W. Geissman New Mexico 87131-1116 ABSTRACT locally derived pebble to boulder conglomer- larly difficult in the Great Basin of the western ate and massive, reworked ash accumulated United States, where workers do not agree on the A study of extension, volcanism, and sedi- in the half graben. These sedimentary rocks tectonic setting and therefore the origin of the ini- mentation in the middle Eocene Panther make up a small part of the basin fill in the tial phases of extension (Gans et al., 1989; Taylor Creek half graben in central Idaho shows that Panther Creek half graben and were derived et al., 1989; Armstrong and Ward, 1991; Best and it formed rapidly during an episode of volumi- mainly from Proterozoic metasedimentary Christiansen, 1991; Axen et al., 1993). A better nous volcanism. The east-southeast–tilted rocks uplifted in the footwall of the basin. The understanding of the diagnostic characteristics of Panther Creek half graben developed across east-southeast tilt of the sedimentary rocks, intra-arc rifts and the variability among true intra- the northeast edge of the largest cauldron their provenance and coarse grain size, and arc rifts will facilitate their identification and may complex of the Challis volcanic field and along the presence of a gravity slide block derived provide new interpretations of their tectonic evo- the northeast-trending Trans-Challis fault from tilted volcanic rocks in the hanging wall lution. Ultimately, such studies may help to zone. Two normal fault systems bound the east attest to continued tectonism during con- resolve conflicting interpretations of extension in side of the half graben. One fault system glomerate deposition. Provenance data from the western United States during the mid-Tertiary. strikes northeast, parallel to the Trans-Challis the sedimentary rocks imply that the high- Our analysis of the Eocene Panther Creek half fault zone, and the other strikes north to land in the footwall of the Panther Creek half graben, part of the Challis volcanic field, central northwest. The geometry of the basin-fill de- graben was never thickly blanketed by synex- Idaho (Fig. 1), and a brief survey of other intra-arc posits shows that movement on these two nor- tension volcanic rocks, despite intense vol- rift basins suggests that the Panther Creek half mal fault systems was synchronous and that canic activity. Analysis of the Panther Creek graben could serve as a model for other rifts that both faults controlled the development of the half graben and other intra-arc rift basins formed very rapidly during an episode of intense, Panther Creek half graben. Strikes of the supports previous interpretations that rela- voluminous volcanism. synextension volcanic and sedimentary rocks tive rates of volcanism and subsidence control We examine the structure, paleogeography, are similar throughout the half graben, the proportion of volcanic rocks deposited in and evolution of this unambiguous intra-arc rift whereas dips decrease incrementally upsec- intra-arc rifts. basin to provide general insights into tectonic tion from as much as 60° to less than 10°. Pre- processes of rapidly extending synvolcanic rift vious K-Ar dates and a new 40Ar/39Ar plateau INTRODUCTION basins. We first describe the geology of the Pan- date from the youngest widespread tuff in the ther Creek half graben, a middle Eocene synvol- basin suggest that most of basin formation The structure, paleogeography, and evolution canic extensional basin. Building on the work of spanned 3 m.y. between about 47.7 Ma and of intra-arc rift basins vary and are difficult to in- Ekren (1985, 1988), we investigate the volcanic 44.5 Ma. As much as 6.5 km of volcanic and terpret (Tobisch et al., 1986; Smith et al., 1987; and sedimentary fill of the basin and the basin- sedimentary rocks were deposited during that Busby-Spera, 1988; Riggs and Busby-Spera, bounding normal faults to determine (1) duration time. Although rates of extension and subsi- 1990; Schermer, 1993; Smith and Landis, 1995). of extension, (2) relative age of faulting and vol- dence were very high, intense volcanic activity Interdisciplinary research furthers our under- canism, (3) rates of extension and subsidence, continually filled the basin with ash-flow tuffs, standing of tectonic processes in the intra-arc set- and (4) the paleogeography of this intra-arc outpacing subsidence and sedimentation, un- ting and develops reliable criteria to discriminate basin. By comparing the diagnostic characteris- til the end of basin development. between intra-arc rifts (synvolcanic) and exten- tics of the Panther Creek half graben to those of After the abrupt end of Challis volcanism, sional basins that form before or after volcanic other intra-arc extensional basins, a simple activity (prevolcanic and postvolcanic). The iden- model is developed to explain some of the vari- *E-mail address: [email protected] tification of synvolcanic rifting has been particu- ability among intra-arc rifts. Data Repository item 9715 contains additional material related to this article. GSA Bulletin; March 1997; v. 109; no. 3; p. 253–267; 8 figures; 2 tables. 253 Downloaded from https://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/109/3/253/3382580/i0016-7606-109-3-253.pdf?casa_token=OeTzwBLfVwoAAAAA:vtEGYkNxtN5SyvS-C9EMdH-uH44bPXCo-uqx_2Jgmwf7qp9MG8sA_NTdSXuT3Qq2gwNakthZ1Q by California Geological Survey, 19774 on 01 April 2020 Figure 1. (A) Map of the faults, dikes, and calderas associated with the middle Eocene Challis and Absaroka volcanic fields, Idaho, Montana, and Wyoming. Synvolcanic normal faults have a consistent northeast strike and are often associated with swarms of northeast- striking dikes. The northeast-striking struc- tural grain may record rapid northeast con- vergence of oceanic plates relative to North America (Janecke, 1992). Some normal faults with northwest (Hope fault; Fillipone et al., 1995) to north (Bitterroot mylonite zone; Hodges and Applegate, 1993) strikes also formed at this time, but some of the deforma- tion along them may be younger. Strike-slip A deformation along the Lewis and Clark fault zone complicates the pattern in the northern part of A. Compiled from Bond (1978); Des- marais (1983); Fillipone et al. (1995); Fisher et al. (1992); Harms and Price (1993); Hyndman et al. (1988); Janecke (1992); King and Beik- man (1974); Lewis and Kiilsgaard (1991); Lewis et al. (1990, 1992a, 1992b); M’Gonigle and Dalrymple (1993); Ruppel et al. (1993); Schmidt et al. (1994); Skipp (1988); Snider (1995); Toth (1987); and Worl et al. (1991). (B) Regional map of the main calderas of the Chal- lis Volcanic field, the Trans-Challis fault zone (TCFZ) and younger normal faults. BCP— Bighorn Crag pluton; CC—Corral Creek cauldron segment; CG—Custer graben; CP— Casto pluton, CR—Castle Rock cauldron seg- ment; KG—Knapp Creek graben; PC—Pistol Creek dike swarm; PCHG—Panther Creek half graben; TM—Thunder Mountain caul- dron complex; TP—Twin Peaks caldera; VH—Van Horn Peak cauldron complex. Sources: Shockey (1957); Bennett (1977); Har- rison (1985); Ekren (1988); Janecke (1994); Evans and Connor (1993); Fisher et al. (1992); Janecke and Snee (1993); Modreski (1985). B 254 Geological Society of America Bulletin, March 1997 Downloaded from https://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/109/3/253/3382580/i0016-7606-109-3-253.pdf?casa_token=OeTzwBLfVwoAAAAA:vtEGYkNxtN5SyvS-C9EMdH-uH44bPXCo-uqx_2Jgmwf7qp9MG8sA_NTdSXuT3Qq2gwNakthZ1Q by California Geological Survey, 19774 on 01 April 2020 RAPID EXTENSION IN AN ACTIVE VOLCANIC ARC Figure 2. Geologic map of Panther Creek half graben. Cross sections are in Figure 4. Sources: Hammond (1994), Ekren (1988), and Bennett (1977). GEOLOGY OF THE PANTHER CREEK zone of high-angle normal faults, dike swarms, A northeast- and a northwest-striking system HALF GRABEN grabens, and elongate plutons that formed in the of normal faults bound the eastern edge of the middle Eocene Challis volcanic field (Fig. 1) Panther Creek half graben (Fig. 2) (Ekren, 1985, About 48 Ma the Panther Creek half graben (McIntyre et al., 1982; Bennett and Knowles, 1988; Fisher et al., 1992; Hammond, 1994). Al- began to form across the northern edge of the 1985; Ekren, 1981, 1985, 1988; Fisher, 1985; though a small antithetic normal fault is also Van Horn Peak cauldron complex, the largest Hardyman and Fisher, 1985; Fisher et al., 1992). present along part of the western margin of the cauldron complex of the Challis volcanic field The Trans-Challis fault zone and other north- basin, the overall structure of the basin is that of (Figs. 1B and 2) (McIntyre et al., 1982; Ekren, east-striking normal faults and dike swarms in an east-southeast–dipping half graben. 1981, 1985, 1988; Fisher et al., 1992; Ham- the region formed in an intra-arc setting during Folded Middle Proterozoic rocks surround the mond, 1994). The Panther Creek half graben is rapid northeast subduction of oceanic plates un- basin on three sides, (Yh, Yy, and Yaq, Fig. 2), the northeasternmost and youngest extensional der the Pacific Northwest (Fig. 1A) (Janecke, whereas intra-caldera tuffs are present