Structure and Paleogeography of an Intra-Arc Half Graben in Central Idaho

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-southeastÐtilted 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. 253Ð267; 8 figures; 2 tables.

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254 Geological Society of America Bulletin, March 1997

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-southeastÐdipping 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 in the basin along the northeast-trending Trans-Challis 1992). Younger episodes of extension have not southwest hanging wall (Tc, Fig. 2). The Panther fault zone (Fig. 1) (McIntyre et al., 1982; Ekren, modified the Panther Creek half graben (Ben- Creek half graben extends about 15 km east- 1985, 1988; Hardyman, 1989), a 270-km-long nett, 1986; Hammond, 1994). west and 20Ð25 km north-south. If our interpre-

Geological Society of America Bulletin, March 1997 255

tation of the tuff of Castle Rock is correct (see Synextension volcanic rocks make up most of considered by Ekren (in McIntyre et al., 1982) to below), the Panther Creek half graben may be a the fill in the Panther Creek half graben and strike be correlative to the tuffs of Challis Creek, but subbasin of a larger northeast-trending half north-northeast and dip 60¡Ð10¡ESE. They in- Ekren (1985) later separated the tuffs of Castle graben with dimensions of 15 km by 40 km. clude from base to top, the dirty tuffs (Tdt), alkali Rock from the tuffs of Challis Creek because the Tertiary rocks in the Panther Creek half feldsparÐplagioclase tuff (Tap), quartz biotite tuff tuffs of Castle Rock consistently contain more graben are as much as 7.6 km thick along the (Tqb), tuffs of Castle Rock (Tck) (Ekren, 1988), lithic fragments and more biotite than do the southeast margin of the basin and are dominated and the tuff of Porphyry Ridge (Tpr, Fig. 2) tuffs of Challis Creek. The tuffs of Castle Rock by north-northeastÐstriking and east-southeastÐ (Hammond, 1994). Each is a distinctive ash-flow were thought to be older than the ca. 46.5 to dipping ash-flow tuffs and lava flows (Ekren, tuff or sequence of similar ash-flow tuffs and 45.5 Ma tuffs of Challis Creek (see Hardyman 1988; Hammond, 1994). Thin (~800 m thick) most had source areas south and west of the Pan- and Snee, cited in Fisher et al., 1992; Janecke sedimentary rocks are concentrated at the top of ther Creek half graben (McIntyre et al., 1982; and Snee, 1993). Because we consider the litho- the section. Small rhyolitic dikes, sills, plugs, Ekren, 1985, 1988; Hammond, 1994). Localized logic differences between these two tuffs to be and laccoliths are abundant along the western basaltic lava flows are also present in the section. small, we revisit their possible correlation. Our and southwestern edge of the basin (Ekren, In contrast to some of the preextension volcanic use of the term “tuffs of Challis Creek” is a mod- 1988; Hammond, 1994). We briefly describe the rocks, the synextension volcanic rocks do not ification of previous nomenclature based on the basin stratigraphy, from oldest to youngest, and crop out in the footwall adjacent to the half gra- presence of at least three flow units of the tuff of then analyze the structure of the graben. More ben (Ekren, 1988). Challis Creek in the Twin Peaks caldera (Hardy- detailed descriptions of the rock units in the Pan- The dirty tuffs and the alkali feldsparÐplagio- man, 1985, 1989; Fisher et al., 1992). ther Creek half graben were in Ekren (1988), clase tuff of Ekren (1988) form the stratigraphi- The tuffs of Castle Rock and tuffs of Challis Fisher et al. (1992), and Hammond (1994). cally lowest, unbroken units across the Panther Creek are considered correlative, on the basis of Creek half graben (Tdt, Tap, Fig. 2), and reach (1) the distribution of the tuffs, (2) new 40Ar/39Ar STRATIGRAPHY OF THE BASIN-FILL maximum thicknesses of 350 and 500 m, re- age determinations and polarity data, (3) the DEPOSITS spectively (Hammond, 1994). The dirty tuffs are structure of the Castle Rock trap-door caldera, undated but are probably 47.6 to 47.7 Ma, on the and (4) the great thickness of the tuffs of Castle Volcanic Rocks basis of the ages of overlying and underlying Rock in the Panther Creek half graben. South- rocks (Fisher et al., 1992). The quartz-biotite tuff west of the Panther Creek half graben, field rela- We distinguish between preextension and of Ekren (1988) extends continuously across the tions (Fisher et al., 1992) show that either the synextension volcanic rocks on the base of dip Panther Creek half graben and reaches a maxi- tuffs of Castle Rock and the tuffs of Challis patterns and their distribution outside the half mum thickness of 700 m and a minimum thick- Creek are the same unit, or that they must in- graben. Preextension lava flows and tuffs pre- ness of 100 m at the present level of exposure terfinger. On the north side of the South Fork of date development of the Panther Creek half (Hammond, 1994). The tuffs of Castle Rock Camas Creek, the essentially flat-lying tuffs of graben and are concentrated along the western (Tck, Fig. 2) (Ekren, 1985, 1988) (within which Castle Rock cap a ridge down to 2400 m. On the margin of the basin (Ekren, 1981; McIntyre et we include with the genetically related mono- south side of the canyon, the flat-lying tuffs of al., 1982). Attitudes of the preextension rocks lith-forming tuff [Ekren, 1988]) are volumetri- Challis Creek also cap a ridge to 2400 m eleva- are more variable than those of the synextension cally the largest synextension volcanic unit in the tion. Both units overlie the quartz-biotite tuff, rocks (Ekren, 1988; Hammond, 1994) because Panther Creek half graben. Because multiple and no faults separate these exposures. they underwent an additional episode of tilting flow units are present in this unit (Ekren, 1988, Two new 40Ar/39Ar age determinations from during collapse of the caldera filled by the tuffs Hammond, 1994), we refer to this unit as the the top of the densely welded part of the tuffs of of Camas CreekÐBlack Mountain. tuffs of Castle Rock. At the present level of ex- Castle Rock in the Panther Creek half graben Preextension volcanic rocks are about 51 Ma posure the rhyolite tuffs of Castle Rock are as (45.4 ± 0.1 [1 sigma] Ma plateau date on sani- to younger than 48.4 Ma and include, from old- thick as 1700 m thick near the southern end of dine) and outflow of the tuffs of Challis Creek at est to youngest, dacite lava flows (Td), the tuff of the basin, and only 300 m thick near the northern Challis Idaho (45.7 ± 0.1 [1 sigma] Ma plateau Ellis Creek (Te), the tuffs of Camas CreekÐ end (Hammond, 1994). We infer that the tuffs date on sanidine) show that these tuffs are about Black Mountain (Tc), and intercalated basalt thicken down dip. At the north end of the basin, the same age (Table DR11 and Fig. 3), in contrast flows (Tb, Fig. 2) (Ekren, 1981, 1988; Fisher et the tuffs of Castle Rock contain abundant lithic to previous interpretations. The normal polarity al., 1992). They are exposed in discontinuous clasts derived from the Yellowjacket Formation of both tuffs (see footnote 1) is consistent with outcrops along the western edge of the basin. in the footwall of the basin (E. B. Ekren, 1994, the age determinations and with our proposed The tuffs of Camas CreekÐBlack Mountain are a written commun.). Overlying the tuffs of Castle correlation (see Appendix). Remanence direc- cauldron-filling sequence at least 3000 m thick, Rock is the newly recognized rhyolitic tuff of tions, however, from the two tuffs are statisti- almost entirely confined to the cauldron com- Porphyry Ridge (Hammond, 1994), as much as cally distinguishable (see Appendix). Different plex in the southwestern part of the Panther 200 m thick, preserved in the northern end of the remanence directions probably reflect one or Creek half graben, that were emplaced before Panther Creek half graben (Fig. 2). more of the following: small errors in the tilt cor- development of the Panther Creek half graben Tuffs of Castle Rock: Regional Correlation rections, minor remagnetizations, sampling of (Ekren, 1981; McIntyre et al., 1982; Ekren, and Source Caldera. Because a large fraction different cooling units, or sampling of different 1985). The tuff of Ellis Creek has an apparent of the slip on the basin-bounding faults occurred flow units. age of 48.4 Ma (K-Ar on biotite, Fisher et al., during deposition of the tuffs of Castle Rock (see 1992), and has particular structural significance below), it is important to determine their re- 1GSA Data Repository item 9715, Table DR1, is because it is preserved in both the footwall and gional stratigraphic setting and age. The tuffs of available on request from Documents Secretary, GSA, hanging wall of the Panther Creek half graben Castle Rock in the Panther Creek half graben P.O. Box 9140, Boulder, CO 80301. E-mail: editing (Ekren, 1988). and the Castle Rock structure were originally @geosociety.org.

256 Geological Society of America Bulletin, March 1997

1988; Hammond, 1994), which locally grade into one another (Ekren, 1988). The intrusions post- date most of the volcanic rocks in the basin and cut volcanic rocks as young as the 45.4 Ma tuffs of Castle Rock and Challis Creek. K-Ar age determinations on sanidine from a quartz porphyry Figure 3. 40Ar/39Ar spec- and from a rhyolite intrusion of 44.4 ± 1.0 Ma tra from three densely and 44.6 ± 1.5 Ma, respectively (McIntyre et al., welded rhyolitic ash-flow 1982; Ekren, 1988; Fisher et al., 1992), imply tuffs. Data are in Table DR1 emplacement shortly after eruption of the com- (see footnote 1 in text). The positionally similar tuffs of Castle Rock and sample localities for the Challis Creek. We infer that the undated intrusive tuffs of Castle Rock and rocks are also about 44.5 Ma. Some of these Challis Creek (II) and the dikes, plugs, laccoliths, and sills may merge at fractured rhyolite ash flow depth with an eastern extension of the granitic tuff (III) are in Figure 2. Casto pluton exposed southwest of the basin The sample locality for the (Ross, 1934; McIntyre et al., 1982). tuffs of Challis Creek (I) is in Figure 1B. Sedimentary Rocks and Fractured Ash- Flow Tuff Masses

Conglomerate and Tuffaceous Sediments. Volcanic rocks of the Panther Creek half graben are overlain by poorly exposed sedimentary rocks (Ts) and fractured masses of ash-flow tuff (Tvl1 and Tvl2, Fig. 2) (McIntyre et al., 1982; Ekren, 1988; Fisher et al., 1992; Hammond, 1994). The sedimentary rocks consist largely of The Castle Rock trap-door caldera is the in- More than 1300 m of the tuffs of Challis Creek boulder and cobble gravels and conglomerate ferred source for the tuffs of Castle Rock (Ekren, are preserved in the Twin Creeks caldera (Fisher beds interlayered with massive tuffaceous sedi- 1985), whereas the Twin Peaks caldera, 40 km et al., 1992). ments, and previously unmapped masses of frac- south-southwest of the Panther Creek half All of these observations are problematic for tured ash-flow tuff (Fig. 2) (Hammond, 1994). graben, was the source for the tuffs of Challis the revised stratigraphy of Ekren (1985) and indi- The conglomerate and tuffaceous sediments Creek (Fig. 1) (Hardyman, 1985; McIntyre et al., cate that McIntyre et al. (1982) may have been form few outcrops, and underlie rolling hills. 1982). The incremental decrease upsection in correct when they originally mapped the tuffs of Massive tuffaceous sediments intercalated in the the dip of volcanic rocks in the Castle Rock Castle Rock and the tuffs of Challis Creek as one conglomerates have been altered to clay and structure, from 71¡ at the base of the Tertiary unit. Alternatively, it is possible that two tuffs tuffaceous subunits are as thick as 10 m. It was section to 18¡ at the top (data from Ekren, 1988), with the same magma source erupted nearly si- difficult to estimate the relative proportion of indicates that this southeast-dipping half graben multaneously from two different calderas. Fur- tuffaceous sediment and conglomerate beds be- did not form in response to the eruption of the ther geochemical, petrologic, stratigraphic, and cause outcrops are generally lacking. The sedi- tuffs of Castle Rock. These relationships suggest structural studies are needed to test our correla- ments are at least 800 m thick along the southeast instead that tilting and faulting had begun about tion and to assess the origin of the Castle Rock and east margin of the basin (Fig. 4, A and B). 2 m.y. before the tuffs of Castle Rock were em- structure. In either case, our new age determina- The only outcrops of cemented conglomerate placed. The 630 m maximum thickness of the tions, when combined with previous work sum- beds consist of moderately to poorly sorted boul- tuffs of Castle Rock in the Castle Rocks trap- marized in Janecke and Snee (1993) and M’Gon- ders and cobbles and expose the stratigraphically door caldera (Ekren, 1985) is considerably less igle and Dalrymple (1993), support the model lowest beds in the west-central part of the sedi- than the 1700 m thick section exposed at the sur- that explosive rhyolitic volcanism in the Challis mentary unit. Clasts are mostly rounded to sub- face at the southern end of the Panther Creek volcanic field occurred primarily during a brief rounded and typically display a polymodal half graben (Hammond, 1994). These observa- ≈0.5 to 1.0 m.y. interval at the end of the Challis grain-size distribution that ranges from coarse tions suggest that the Castle Rock structure is volcanic episode. Below, we refer to the tuffs of sand and granules to boulders. Beds are poorly more easily interpreted as a synvolcanic half Castle Rock in the Panther Creek half graben as sorted and clast supported. Cobble imbrication graben than as a trap-door caldera or volcano- the tuffs of Castle Rock and Challis Creek to re- data were collected at four sites at this exposure tectonic depression that formed by collapse of flect our preferred interpretation that the two tuffs to assess the relative importance of the footwall the hanging wall into a partially emptied magma are one unit. and hanging wall as a sediment source and to de- chamber. In the half-graben interpretation, the lineate sediment dispersal patterns. The mean great thicknesses of the tuffs of Castle Rock in Intrusive Igneous Rocks flow directions range from N28¡W to N65¡W the Castle Rock and Panther Creek structures re- with an average flow vector of N43¡W (Fig. 5). flect rapid subsidence along the basin-bounding Intrusive rocks in and adjacent to the Panther Northwestward sediment dispersal in this part normal faults, coincident with eruptions of rhyo- Creek half graben include quartz porphyry and of the conglomerate and tuffaceous sediments is lite ash-flow tuff from the Twin Peaks caldera. rhyolitic plugs, laccoliths, sills, and dikes (Ekren, also suggested by clast composition data col-

Geological Society of America Bulletin, March 1997 257

Figure 4 (A and B). Cross sections of the Panther Creek half graben. No vertical exag- geration. Cross-section AÐA′ is perpendicular to the strike of the basin-filling deposits and approximately perpendicular to the trace of the Rabbit Foot fault system. Intrusions in the western part of the basin may be related to the Casto pluton, but this interpretation of the subsurface is not shown. Cross-section BÐB′ is perpendicular to a north-striking seg- ment of the Moyer Ridge fault. In the hanging wall, units Yh and Yy beneath Yaq were not projected into the section. Unit symbols B are in Figure 2. We determined a cut-off angle of 75¡ between hanging-wall rocks and the basin-bounding faults in the shallow part of the basin and assumed this angle throughout both cross sections. These cross sections furnish minimum estimates of slip on the basin-bounding faults (Table 1).

lected throughout the unit (Fig. 6). The lithology gin (Fig. 6, domain 3). There, clasts of dark, thin- preserved in the hanging wall west of the con- of 50 pebbles, cobbles, and boulders was deter- bedded quartzite-argillite derived from the Yel- glomerate and tuffaceous sediments, but may mined at each site. Clast composition data were lowjacket Formation, Apple Creek Formation, or once have been present in the footwall of the collected from all areas of the conglomerate and argillaceous quartzite unit, dominate the sedi- basin. Quartzite-argillite is mostly widely ex- tuffaceous sediments where beds of cobble and mentary rocks. No coarse-crystalline intrusive ig- posed in the northeast footwall, but is also pres- boulders were evident on aerial photographs. In neous clasts were observed. ent in part of the southeast footwall and north- domain one, at the base of the sedimentary suc- Sediment dispersal patterns are clear because west hanging wall (Fig. 2). cession, volcanic clasts dominate. Clasts of white different rock types characterize the northeast, The abrupt transition from a source area dom- to off white, massive Hoodoo Quartzite dominate southeast, and west edges of the basin (Figs. 2 inated by volcanic rocks (Fig. 6, domain 1) to the remainder of the conglomerate and tuffaceous and 6A). The Hoodoo Quartzite is widely ex- source areas dominated by Proterozoic rocks sediments (Fig. 6, domain 2), except in the upper posed in the southeast footwall and in parts of (Fig. 6, domains 2 and 3) suggests that (1) the stratigraphic levels near the northeast basin mar- the northwest hanging wall. Volcanic rocks are footwall was the major source of sediment, and

258 Geological Society of America Bulletin, March 1997

welded parts of the tuffs of Castle Rock and Challis Creek in crystal content and compositions, color, abundance of lithic and pumice fragments, and degree of welding (Hammond, 1994). Proterozoic clasts derived from the footwall of the Moyer Ridge fault dominate the conglomerate beds beneath the fractured rhyolite tuff, and volcanic clasts are notably absent from these beds (Fig. 6). These relationships indicate that volcanic rocks had been stripped by erosion from the northeast footwall prior to emplace- Figure 5. Paleocurrent ment of the fractured rhyolite tuff. We therefore data from four closely interpret the fractured rhyolite tuff as a Paleo- spaced exposures of the gene rock slide that was emplaced eastward conglomerate and tuffa- from a source in the hanging wall onto stream ceous sediments. See Fig- gravels derived from the footwall. The closest ure 2 for location. Imbri- possible source of the fractured rhyolite tuff in cation orientations were the tuffs of Castle Rock and Challis Creek is found by measuring the more than 5 km west of the site. Using relation- azimuth of the pole to the ships in Topping (1993) and assuming that about plane containing the long 20% of the original volume of the rock slide is and intermediate axes of preserved on Moyer Ridge, yields a horizontal ≈ 25 clasts at each of four runout between 5 and 10 km. sites in the exposed con- A 45.7 ± 0.1 Ma 40Ar/39Ar plateau date from glomerate beds. Results the fractured rhyolite tuff on Moyer Ridge (sani- were not corrected for mi- dine, Fig. 3, Table DR1 [see footnote 1]) is older nor (<10¡) tilting. than the date from the densely welded upper part of the tuffs of Castle Rock and Challis Creek (dated as 45.4 ± 0.1 Ma), about 1 km down section. The dates, which are statistically different at the 99% confidence level based on a two-sample Z test, are in the wrong order for their stratigraphic position. The fractured rhyolite tuff has a normal polarity, like the tuffs of Castle Rock and Challis Creek (see Appendix). The 40Ar/39Ar and polarity data are consistent with the fractured rhyolite tuff being a gravity-emplaced rock slide derived from the lower three-quarters of the tuffs of Castle Rock and Challis Creek, and are incon- (2) the footwall was not thickly blanketed by that during the deposition of conglomerate beds sistent with the fractured rhyolite tuff being an in consolidated volcanic material at the onset of of domain 1a and 1b, (Fig. 6) a paleohill localized place ash-flow tuff. deposition of the conglomerate and tuffaceous deposition to the north and south. This topo- Significance of the Sedimentary Rocks and sediments. Conglomerate beds dominated by graphic high may not have been covered by con- Fractured Ash-Flow Tuff Masses. Magmatism clasts of Hoodoo Quartzite (Fig. 6, domain 2) glomerate until the time when clasts of Protero- and extension both ended during deposition of extend as much as 10 km to the northwest from zoic metasedimentary rocks dominated the basin the conglomerate and tuffaceous sediments. Ash their likely source area along the southeast mar- fill. If we are correct in interpreting the paleohill from distant sources may have been deposited in gin of the basin. Cobble imbrications from do- as a dome above a late-stage laccolith (B in the basin and reworked into massive tuffaceous main 2 (Fig. 6) thus agree with the clast compo- Fig. 2), then initial deposition of the conglomerate sediments, but volcanic activity in the central sition data in showing a southeast to northwest and tuffaceous sediments coincided with the final cauldrons of the Challis volcanic field had ended dispersal of sediment. phases of Challis magmatism about 44.5 Ma. or dropped to a low level after emplacement of Clast composition data also show that a topo- Fractured Rhyolite Ash-Flow Tuff. A highly the tuffs of Castle Rock and Challis Creek graphic high in the center of the basin controlled jointed, densely welded ash-flow tuff that is as (Fisher et al., 1992; Janecke and Snee, 1993). sediment-dispersal patterns at the base of the con- thick as 40 m and has dimensions of nearly 2 km Deposition of the tuff of Porphyry Ridge and in- glomerate and tuffaceous sediments (Figs. 2 by 0.5 km, caps the ridge west of Moyer Creek trusion of quartz porphyry and rhyolite, before and 6). Conglomerate beds dominated by vol- (informally known as Moyer Ridge) (Tvl1, beds of domain two and three of the conglomer- canic clasts are present in two areas (Fig. 6, do- Fig. 2). The basal contact is not well exposed, but ate and tuffaceous sediments were deposited mains 1a and 1b,), divided by an area dominated mapping suggests that the fractured tuff has a tab- (Fig. 6A), are the final magmatic events re- by clasts of Hoodoo Quartzite (Fig. 6, domain 2) ular and horizontal geometry and laps across the corded in the Panther Creek half graben. that were deposited directly on the tuffs of Castle inferred trace of the Moyer Ridge fault (Fig. 2). The conglomerate and tuffaceous sediments Rock and Challis Creek. This pattern suggests The fractured rhyolite tuff resembles the densely were deposited during and slightly after the final

Geological Society of America Bulletin, March 1997 259

Figure 6 (A and B). Provenance data from 52 sites in the conglomerate and tuffaceous sediments. A shows sample sites and boundaries of domains de- lineated on the basis of clast compositions (see B). The unit abbreviations are in Figure 2. B shows the com- positional data and domains 1Ð3.

A B

phases of extension in the Panther Creek half (Ekren, 1988). This, paleocurrents, and the pres- STRUCTURAL EVOLUTION OF THE graben. The east-southeast tilt of the unit, the ence of Proterozoic clasts within 200 m of the PANTHER CREEK HALF GRABEN provenance and large size of many clasts, and the base of the conglomerate and tuffaceous sedi- presence of rock-slide masses show that deposi- ments in the northern half of the Panther Creek Fault Systems tion of most of the conglomerate and tuffaceous half graben (Fig. 6), show that the footwall was sediments was syntectonic to slip on the basin- a positive topographic feature during most of The Rabbit Foot and Moyer Ridge fault sys- bounding faults. However, because the conglom- basin development. The highland east of the tems are the major basin-bounding normal faults erate and tuffaceous sediments locally lap across basin hampered eastward dispersal of ash-flow of the Panther Creek half graben. Normal faults segments of both basin-bounding normal fault tuffs with source areas west of the basin. Either with small offsets (<1 km) are present along the systems, and dip less than 10¡ toward the basin- synextension volcanic rocks were never thickly western margin of the basin in rocks older than bounding faults, they record only the last incre- deposited on the footwall (our preferred inter- the dirty tuffs and along the northern and south- ment of extension in the Panther Creek half gra- pretation) and/or they were continually stripped ern basin margins in rocks younger than the tuffs ben (Figs. 2 and 4). Altogether, these data show by erosion as the basin developed. As a result, of Camas CreekÐBlack Mountain. that extension and volcanism ended at essentially the bulk of the sediment in the basin was derived Faults Along the Western Margin of the the same time in the Panther Creek half graben. from Precambrian metasedimentary rocks that Half Graben. A north-northeastÐstriking nor- The provenance of the conglomerate and tuf- are exposed in the footwall highlands. Volcanic mal fault juxtaposes basin-fill deposits against faceous sediments implies that the footwall of rocks accumulated at extremely high rates Proterozoic rocks along part of the western mar- the Panther Creek half graben was a highland (~2 km/m.y.) (Fisher et al., 1992; Janecke and gin of the basin, but along the northwestern mar- during the second half of the Challis volcanic Snee, 1993; Armstrong and Ward, 1991) in gin, volcanic rocks rest in depositional contact episode. Some preextension ash-flow tuffs and many parts of the Challis volcanic field, yet Prot- on Proterozoic rocks. The north-northeastÐstrik- lava flows are preserved in both the hanging wall erozoic rocks uplifted along the Rabbit Foot and ing normal fault and others in the western part of and footwall of the basin, whereas synextension Moyer Ridge fault systems were at or near the the basin offset rocks older than the dirty tuffs tuffs are absent from the adjacent footwall surface during peak volcanism. (Fig. 2). Most faults strike north-south, but a

260 Geological Society of America Bulletin, March 1997

subordinate set strikes east-west. Displacements sonable cutoff angle (~75¡) with steeply tilted Foot fault system suggests an additional 1.4 km across the faults are generally less than a few hanging-wall rocks. of heave, 2.8 km of throw, and 3.2 km of dip-slip hundred meters, but some have offsets of 1 km. An offset synclinal hingeline developed in pre- displacement (Hammond, 1994). If these addi- Quartz porphyry intruded some of these small Tertiary rocks in the footwall and hanging wall of tional estimates are added to the minimum value faults (Figs. 2 and 4). the Rabbit Foot fault system (D and E in Fig. 2) in Table 1, then as much as 6.8 km of heave, Crosscutting relationships suggest that these also suggests that the fault system is listric. In both 8.6 km of throw, and 11.3 km of total dip-slip dis- faults formed before subsidence began in the areas, a broad southeast- to east-southeastÐtrend- placement may have occurred on the fault sys- Panther Creek half graben. Some of the faults ing syncline is present in the Hoodoo Quartzite tem. Displacement across the Rabbit Foot fault are truncated by the east-westÐstriking margin of northeast of a northwest-striking strike-slip fault system decreases toward the southwest along the Van Horn Peak cauldron complex (C in (Ekren, 1988). The folds were once continuous several northeast-striking splays. The aggregate Fig. 2) (McIntyre et al., 1982; Ekren, 1988), across the basin (Ekren, 1988) and we assume that dip-slip displacement near Singheiser mine is which in turn is overlapped by the oldest synex- the syncline had a fairly uniform plunge before greater than 3 km (Hammond, 1994). tension tuff of the Panther Creek half graben, the normal faulting. If so, the present gentle north- More than 35% of the dip-slip displacement dirty tuffs. The proximity of the small faults to northwest plunge of the fold in the footwall and across the Rabbit Foot fault system occurred the margin of the cauldron complex suggests the gentle east-southeast plunge in the hanging during the deposition of the tuffs of Castle Rock that they may have formed during the collapse of wall reflect differential rotation across the Rabbit and Challis Creek (Table 1 and Fig. 4), whereas the caldera occupied by the tuffs of Camas Foot fault system that is consistent with a listric about 10% of the total offset occurred during CreekÐBlack Mountain. geometry. Ekren (1988) interpreted the Rabbit deposition of the sedimentary rocks at the top of Basin-Bounding Faults. Rabbit Foot Fault Foot fault system as a left-oblique normal fault. the basin-fill sequence. We interpret this dis- System. The northeast-striking and northwest- The projections of the synclinal axes, although placement history to suggest that subsidence and dipping Rabbit Foot fault system juxtaposes a poorly defined, do not support a large left-lateral extension rates may have peaked very near the thick section of east-southeastÐdipping Challis offset across the fault system. end of basin formation and then dropped to low volcanic rocks and sedimentary rocks in the The minimum dip-slip displacement across levels. We thus agree with McIntyre et al. (1982) hanging wall against Proterozoic and caldera- both splays of the Rabbit Foot fault system near that emplacement of the tuffs of Castle Rock and filling rocks in the footwall (Fig. 2). At its north- its northeastern end is 8.1 km (Table 1 and Challis Creek coincided with major subsidence east end the fault system consists of two major Fig. 4A), assuming a listric geometry. This esti- in the Panther Creek half graben. fault splays, which we call the southeast and mate represents the slip needed to restore the Previous studies identified a northeast-strik- northwest splays. Exposures of the northwest base of the stratigraphically lowest Tertiary unit ing, high-angle fault along Panther Creek as an splays are limited because the conglomerate and to horizontal before faulting. The stratigraphy of important tectonic feature of central Idaho tuffaceous sediments cover most of its trace. The the footwall and hanging wall of the Panther (Hughes, 1990; Kiilsgaard et al., 1986; Link et southeast splay, by contrast, cuts the conglomer- Creek half graben differ so much that the basin al., 1993; Bartels et al., 1990) and as the probable ate and tuffaceous sediments. These relation- fails to meet the basic criteria for palinspastic northeast extension of the Trans-Challis fault ships show that the latest slip on the southeast restoration (Davison, 1986; Rowan and Kligfield, zone (Panther Creek fault in Fig. 1; O’Neill and splay of the Rabbit Foot fault system was 1989; Groshong, 1989; Dula, 1991), and the Lopez, 1985; Kiilsgaard et al., 1986). We do not slightly younger than the latest slip on the north- amount of footwall uplift could only be estimated support these previous interpretations because west splay. Southwest of the Rabbit Foot mine, for the tuff of Ellis Creek. Projecting the tuff of we find no evidence for a fault along Panther the Rabbit Foot fault system consists of several Ellis Creek in the footwall across two faults from Creek in the Panther Creek half graben (Fig. 2). smaller northeast-striking fault splays (Fig. 2) as exposures about 10 km southeast of the Panther Moyer Ridge Fault and Related Splays. The it loses displacement to the southwest. The Creek half graben to its intersection with Rabbit Moyer Ridge fault is a north-northwestÐstriking length of the Rabbit Foot fault system is uncer- tain because some displacement may be transferred to the fault bounding the Castle Rock half TABLE 1. MINIMUM ESTIMATES OF HEAVE, THROW, AND DIP-SLIP DISPLACEMENT graben southwest of the study area (Fig. 1B). ACROSS THE MOYER RIDGE FAULT AND RABBIT FOOT FAULT SYSTEM Three-point solutions of mapped fault traces, Unit Heave (km) Throw (km) Total displacement (km) Percent displacement during deposition of each unit observations in the field, and the observations of Moyer Ridge fault other workers indicate a northeast strike and a Te 3.4 3.3 4.9 0 steep northwest dip for most of the splays of the Tdt 3.3 3.2 4.8 23* Rabbit Foot fault system. The northwest splay Tqb 2.5 2.8 3.8 20 Tck 1.7 2.2 2.8 37 has both north- and northeast-striking segments Ts and Tpr .5 .8 1.0 20 (Fig. 2). Umpleby (1913) noted that in the Sing- Rabbit Foot fault system† heiser mine, a splay of the Rabbit Foot fault sys- Td, Te & Tb 5.4 5.8 8.1 0 tem strikes N35¡Ð40¡E and dips 65¡NW. A sec- Tdt & Tap 4.6 5.6 7.3 24¤ ond exposure of the southeast splays of the Tqb 3.7 4.7 6.2 <30 Tck >2.2 >3.0 >3.8 >35 Rabbit Foot fault system is nearly vertical and Ts 0.4 0.8 0.9 11 has slightly left lateral, oblique-slip slickenlines, Note: Values determined by rotating the base of each unit to horizontal in cross section BÐB′ and with rakes of 80¡ to the southwest (Hammond, AÐA′ (Fig. 4). These are minimum values only. The amount of displacement in the footwall can not be 1994). We infer that the Rabbit Foot fault system accurately determined. Unit symbols in Fig. 2. *Includes offset of Te. must have a listric geometry at depth to satisfy †Sum across two fault strands. the steep dip at the surface and to maintain a rea- ¤Includes offset of Td, Te and Tb.

Geological Society of America Bulletin, March 1997 261

normal fault along the northeast flank of Panther Creek half graben (Fig. 2). At its northwest end, displacement across the fault decreases and may be transferred to a series of small-offset north- and northwest-striking normal faults (Fig. 2). The overall strike of the Moyer Ridge fault is roughly perpendicular to that of the Rabbit Foot fault system (Fig. 2). The Moyer Ridge fault juxtaposes a thick section of east-southeastÐdipping Challis volcanic rocks in the hanging wall against the Proterozoic Yellowjacket Formation in the footwall. The Moyer Ridge fault is covered by Quater- nary deposits and upper units of the conglomerate and tuffaceous sediments along much of its trace. Three-point solutions of the fault’s probable map trace indicate a north to northwest strike and a 55¡Ð65¡, west to southwest dip (Ham- mond, 1994). Dip-slip displacement across the Moyer Ridge fault is more than 5 km, and almost 40% of the displacement occurred during deposition of the tuffs of Castle Rock and Challis Creek (Table 1, Fig. 4). The minimum estimate of dip- slip displacement is 60% of that across the Rab- bit Foot fault system (Table 1). The fault must have a listric geometry at depth to maintain rea- sonable cutoff angles (~75¡) with hanging-wall rocks. The proportion of strike-slip to dip-slip displacement across the Moyer Ridge fault is unknown. The high-angle intersection of the Moyer Ridge and Rabbit Foot fault systems could re- σ flect either near uniaxial regional extension ( 2 σ approximately equal to 3) or reactivation of preexisting planes of weakness. We prefer the second explanation because the consistent northeast strike of faults and dikes along the Trans- Challis fault zone (Kiilsgaard et al., 1986) and throughout the region (Fig. 1A) indicates a σ northwest-trending 3 that is distinctly smaller σ than 2. The basin-bounding faults may have reactivated prevolcanic(?) northeast-and north- westÐstriking, strike-slip faults exposed in Prot- erozoic rocks northwest and southeast of Panther Creek half graben (Figs. 1 and 2) (McIntyre et al., 1982; Ekren, 1988; Hammond, 1994). The evidence for reactivation is particularly clear for the Moyer Ridge fault, which is along strike of a preexisting northwest-striking strike-slip fault (Fig. 2).

Tilting of the Basin-Fill Deposits Figure 7. Lower-hemisphere equal-area stereograms showing poles and great circles of bed- The dip of synextension volcanic rocks de- ding in the sedimentary units (Ts) and compaction foliations in the volcanic rocks (Tck, Tqb, creases gradually upsection, from an average Tap, Tdt, Te). Symbols as in Figure 2. Stereograms are generated from measurements of strikes dip of 54¡ in the alkali feldsparÐplagioclase and dips in Hammond (1994) and Ekren (1988). The “All rocks” lower-hemisphere equal-area tuffs and dirty tuff to 8¡ in the conglomerate and stereogram shows the average pole to bedding for each stratigraphic unit. Note the progressive tuffaceous sediments (Figs. 4 and 7). The atti- increase in dip from the youngest (Ts) to the oldest rocks (Te, Tap, Tdt), and the consistent east- tude of the tuff of Ellis Creek, a preextension southeast dip direction.

262 Geological Society of America Bulletin, March 1997

unit, is statistically indistinguishable from that Rates of Extension and Relation to M’Gonigle and Dalrymple, 1993) probably ex- of the first synextension tuffs (Tdt, Tap, and Volcanism plains the high strain rates in the Panther Creek Tqb, Fig. 7). The gradual decrease in dip upsec- half graben. tion in the synextension volcanic rocks demon- The long-term average slip rate on faults in strates that extension, tilting, and volcanism the Panther Creek half graben is not well known DISCUSSION AND SUMMARY OF THE were synchronous in the Panther Creek half because the youngest slip along the Rabbit Foot PANTHER CREEK HALF GRABEN graben and began after deposition of the tuffs of and Moyer Ridge fault systems is undated. We Camas Creek and Black Mountain (see also therefore estimate slip rates during deposition of The east-southeast tilt of the Panther Creek half McIntyre et al., 1982). the tuffs of Castle Rock and Challis Creek and graben has been attributed to the diapiric ascent of The strikes of the synextension deposits are compare this likely maximum slip rate with esti- the Casto pluton along the southwest margin of oblique to both basin-bounding normal faults mates of the long-term average slip rates that are the basin (McIntyre et al., 1982; Leonard and (Figs. 2 and 7), differing about 25¡ from the av- based on the assumption that deposition of the Marvin, 1982; Hardyman, 1989). The pluton in- erage strike of the Rabbit Foot fault system and tuff of Porphyry Ridge and the conglomerate trudes older volcanic rocks of the Challis volcanic about 45¡ from the average strike of the Moyer and tuffaceous sediments spanned 2Ð3 m.y. The group, but it is compositionally and probably ge- Ridge fault. This geometry indicates that the available age determinations indicate that the netically related to the youngest widespread ash- Rabbit Foot fault system is the larger normal tuffs of Castle Rock and Challis Creek were em- flow tuffs of the Challis volcanic group, the rhy- fault (in agreement with our previous estimates) placed between about 45.4 and 46.5 Ma (Fisher olitic tuffs of Castle Rock and Challis Creek and because the component of dip toward the Rabbit et al., 1992; Janecke and Snee, 1993; this study). the Sunnyside tuffs (Bennett and Knowles, 1985; Foot fault system exceeds that toward the Moyer Rates of extension and tilting were extremely Fisher et al., 1992; Larson and Geist, 1995). The Ridge fault. We infer that slip along the two rapid in the Panther Creek half graben. As much age of the pluton is poorly constrained by K-Ar faults occurred simultaneously, because the av- as 55¡ of tilting occurred in about 2 to 4.5 m.y., dates that span about 4 m.y. (47.8 ± 1.9 to 43.9 ± erage strikes of the synextension basin-fill de- yielding rotation rates of about 12¡ to 27.5¡/m.y. 1.3 Ma ) (Fisher et al., 1992). posits are nearly constant even though the aver- Slip rates on the Rabbit Foot fault system, the In contrast, we infer that tilting in the Panther age dips vary as much as 60¡ (Fig. 7). larger of the two basin-bounding fault systems, Creek half graben is a natural consequence of exceeded 2.9 km/m.y. during deposition of the extension along two large listric normal faults: Age of Extension tuffs of Castle Rock and Challis Creek and aver- Rabbit Foot and Moyer Ridge fault systems. Be- The Moyer Ridge and Rabbit Foot fault sys- aged 2.0 ± 0.4 km/m.y. (slip was distributed cause the Casto pluton does not persist beyond tems probably initiated during deposition of the across two fault splays). The Moyer Ridge fault the northern margin of the Van Horn Peak caul- 47.6 to 47.7 Ma dirty tuffs (McIntyre et al., had slip rates in excess of 1.6 km/m.y. during dron complex, its diapiric ascent cannot explain 1982). The age of final movement on the basin- deposition of the tuffs of Castle Rock and Chal- tilting in the northern half of the Panther Creek bounding faults is less well known because the lis Creek, but averaged about 0.7 to 1.0 km/m.y. half graben (Fig. 1B). The north-northeast strike conglomerate and tuffaceous sediments are un- Basin fill accumulated at rates that exceeded and east-southeast tilt direction of volcanic rocks dated, but we infer that slip continued for at most 2.9 km/m.y. during deposition of the multiple in the basin, however, are consistent across the a few million years after the 45.4 ± 0.1 Ma tuffs flows of the tuffs of Castle Rock and Challis entire basin (Figs. 2 and 7; Hammond, 1994). of Castle Rock and Challis Creek were em- Creek, and long-term rates averaged about 1.4 to The elongate form and northeast trend of the placed. The basin was almost fully formed about 2.9 km/m.y. near the Rabbit Foot fault system. Casto pluton are consistent with emplacement 44.5 Ma when rhyolite and quartz porphyry These rates are greater than rates documented in along northeast-striking faults or fractures paral- dikes, sills and plugs, possibly related to the most other intra-arc basins (see Table 2). lel to the Trans-Challis fault zone. A fracture- Casto pluton, intruded the splays of the Rabbit Rates of subsidence and extension peaked near controlled emplacement model is supported by Foot fault system and the western part of the the end of basin formation and apparently the coincidence of apophyses at the northeast basin. Renewed activity on the Rabbit Foot fault dropped to low levels during deposition of the and southwest ends of the pluton with the mar- fractured the intrusions and facilitated mineral- conglomerate and tuffaceous sediments. These gins of the once continuous Van Horn Peak and ization (McIntyre et al., 1982), but slip must peak rates of subsidence and extension coincided Thunder Mountain cauldron complex (Fig. 1B; have been minor because vertical quartz por- with the last major magmatic event in the Challis Hardyman, 1989; Fisher et al., 1992). The pluton phyry dikes in the western part of the basin were volcanic field, the eruption of rhyolite ash-flow may have utilized these preexisting weaknesses not appreciably tilted (Fig. 4A). These dikes cut tuffs and emplacement of granite plutons. Al- to spread out in a northwest-southeast direction, 50¡ to 60¡ east-southeastÐdipping volcanic rocks though extension in the Panther Creek half resulting in its irregular “I” shape (Fig. 1B). and preexisting north-northeastÐstriking normal graben ended shortly after the end of volcanism, Our studies agrees with previous interpreta- faults. We thus infer that most of the tilting of the there is no correspondence between initial exten- tions that development of the Panther Creek half Panther Creek half graben occurred prior to the sion and early phases of Challis volcanic activity. graben was synchronous with volcanism in the emplacement of these dikes and that the basin The Panther Creek half graben began to form af- Challis volcanic field (McIntyre et al., 1982; was nearly fully developed at the end of Challis ter the largest caldera-forming eruptions in the Ekren, 1985, 1988; Fisher et al., 1992). Ex- magmatism. field, the formation of the Van Horn Peak caul- tremely high rates of volcanic activity in the field Only about 10% to 20% of the net slip on the dron complex, and about 2Ð3 m.y. after initial continually filled the subsiding half graben with basin-bounding faults occurred after 44.5 Ma volcanism (Fisher et al., 1992). The negligible tilt ash-flow tuffs. Extension began 2 to 3 m.y. after during deposition of the upper part of the con- of the youngest basin-fill deposits suggests that the earliest volcanism in the field and only 10% glomerate and tuffaceous sediments. Extension magmatic activity did not trigger postvolcanic to 20% of extension continued after magmatism ended shortly after magmatism and most of the extension. The great intensity and short duration in the region ceased. During the final phases of extension occurred in 2 to 3 m.y., between about of Challis magmatic activity (McIntyre et al., magmatism in the basin, felsic plugs invaded 47.7 and 44.5 Ma. 1982; Ekren, 1985; Janecke and Snee, 1993; one of the basin-bounding faults, vertical dikes

Geological Society of America Bulletin, March 1997 263

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 JANECKE ET AL. d, t d, 1994, ce e et al., 1982; e and Sour , 1981, 1990; n, 1985; Ewar y Smith, 1994, en, 1988 amaji, 1990 aylor T Smith et al., 1987; personal commun. Priest, 1990; Smith and Landis, 1995; Gar personal commun. Allan, 1986; Righter et al., 1995 Geist et al., 1988; This study; Ster Scholl et al., 1975 McIntyr Ekr et al., 1977; Cole Carlson and Moye, McIntyr Johnson, 1985; Fisher et al., 1992; Allen and Hahn, 1994 1979; Smith and Landis, 1995 Y 1990; Meussig, 1962, 1967; Staatz, 1964; Pearson and Obradovich, 1977; Mathews and Gaylor 1994; Suydam, 1993; David Gaylor , , late . S >> V Relative rate of subsidence (S) aries, S >= V and volcanism (V) S >> V at 5.4 Ma. Later V >=S S > V (?) S > V S = V until S>= V S = V volcanism stopped early S = V V . . . . emely rapid . . up to . (0.5 to 2 mm/yr) ., Extr m average = m rate = m rate=about 6.5 km n extension Accumulation rate the rate of subsidence) (mm/yr) (an estimate of Long-ter 2–4 km / 5.4 to 4 m.y (0.4–1 mm/yr) 2.5 km/11–12.5 m.y (~0.2 mm/yr) Up to ~4 km/ 5–6 m.y Long-ter 2 km /4 m.y (<1 mm/yr) (6%–13% extension) 1.4–2.9 mm/yr; peak rate exceeded 2.9 mm/yr) Rapid (duration only a (Long-ter in 2–5 m.y >2 km/m.y (1–3 mm/yr) few million years) 1–3 km/m.y rates of 7–4 mm/yr Moder (>0.6 mm/yr) (Footwall uplift Rapid, 3.5 km / < 6 m.y has rates of ~1 mm/yr) mities mities y mities mities (?) mities olcanics es owth faulting, y of the rift owth faulting, ominent l uctur mable with some Geometr owth str and basin-fill deposits apering half graben, Graben, mostly confor angular unconfor Graben and half graben T Half graben, gr Graben gr Asymmetric graben, and angular unconfor and angular unconfor angular unconfor Graben, gr during early histor angular unconfor Graben, pr above Sanpoil V c elative to ar y oblique to y oblique to Orientation of er er Parallel Both parallel and perpendicular Subparallel V Parallel V nearly perpendicular nearly perpendicular Subparallel Paralle basin r CTERISTICS OF SELECTED INTRA-ARC RIFTS ocks, fs) ocks CHARA ocks ocks ocks y r ocks, y r y r eworked ash) e mafic fs, and ABLE 2. ocks ocks and some hyolitic ash-flow T ocks (ash, ocks (mostly ocks (mostly lava ocks (lava flows fs, some lava flows, y and volcanic r y r y r y r y r faceous sandstone and ocks and minor ocks (r ocks and minor ocks) eas the sedimentar Basin fill omagmatic tuf fs and some mor olcanic r olcanic r olcanic r ock avalanches) and some sedimentar (basalt lava flows, Mostly volcanic r Sedimentar (lacustrine deposits floor the hydr lacustrine deposits) pelagic sediments, and graben, localized volcanic centers) Sedimentar conglomerate, and r sedimentar tuf V volcaniclastic sands, silts, and muds) V sedimentar ash-flow tuf V volcanic r overlain by postrift marine mudstones and shales nonmarine sedimentar and 15%–30% volcaniclastic Roughly 70%–85% volcanic r flows and some ash-flow tuf Synrift volcanic r sedimentar dominate the volcanic r wher include tuf r shale, volcanic conglomerate, and c c c c c c c c Setting Continental ar on oceanic basement Continental ar Island ar Continental ar Continental ar Continental ar massif Continental ar Island ar Age Late Cenozoic Late Cenozoic Late Cenozoic Cenozoic Eocene Late Eocene (18–15 Ma) Eocene Miocene n egion c eek half graben c th Island of olcanic belt olcanic r egon Cascade Range High Cascade graben of the Or Location Colima, Chapala, and Zacoalco grabens of the Mexican V Summit basins of the central and wester in the Challis volcanic field on the Nor Central V Aleutian ar Challis volcanic field Custer graben in the New Zealand Panther Cr Sanpoil volcanic field Republic graben in the Japanese ar

264 Geological Society of America Bulletin, March 1997

cut tilted rocks in the hanging wall, and laccoliths folded the tuffs of Castle Rock and Challis Creek. Rates of extension, subsidence, and tilting were also high. The main pulse of extension coincided with eruption of the tuffs of Castle Rock and Challis Creek near the end of basin formation. However, magmatism did not trigger further extension, as hypothesized for other areas in the Cordillera (Armstrong and Ward, 1991), because little normal faulting continued after the end of volcanic activity. The exposed basin fill consists almost entirely of ponded ash-flow tuffs. Sediment began to ac- cumulate in the half graben after volcanism waned. The sediment was derived mostly from Proterozoic rocks uplifted in the footwall to the southeast and northeast, but contains at least one intercalated rock slide derived from tilted volcanic rocks in the hanging wall. Volcanic debris reworked from the underlying volcanic rocks makes up a small part of the sedimentary section. Figure 8. Semiquantitative graph of relative rates of volcanism versus accumulation in se- Volcanism in the Challis volcanic field was so lected intra-arc rift basins. Sources of data in Table 2. M—Mexican volcanic belt; PC—Pan- voluminous and widespread that most preexist- ther Creek half graben. Asterisk denotes submarine rift basins. ing topography was blanketed and buried by volcanic rocks by the end of this volcanic episode (Armstrong, 1978; Ekren, 1988; Janecke and no evidence for the Panther Creek fault in the half Cascade graben, which appears to be filled by Snee, 1993). East of the Panther Creek half graben, a northeast-striking fault that is thought to about 2 to 4 km of Neogene basalt flows and graben, however, normal faulting and footwall be a major tectonic feature in central Idaho. some volcaniclastic rocks (Taylor, 1981, 1990; uplift produced highlands exposing Proterozoic Smith et al., 1987; Priest, 1990; G. A. Smith, rocks that had at most a thin cover of volcanic COMPARISON OF THE PANTHER 1994, personal commun.), and the Central Vol- rocks at the end of Challis volcanic activity. The CREEK HALF GRABEN WITH OTHER canic region on the North Island of New Zea- highlands persisted to the end of Challis volcan- INTRA-ARC BASINS land, which contains thick sections of ash-flow ism, reduced eastward dispersal of the ash-flow tuffs and pyroclastic-rich sediments (Stern, tuffs in the Challis volcanic field, and may ex- In order to evaluate whether the Panther Creek 1985; Ewart et al., 1977; Cole, 1979; Wilson et plain the apparent absence east of the highland half graben is typical of other intra-arc rifts and to al., 1984; Smith, 1991; Smith and Landis, 1995; (Fisher et al., 1992) of voluminous ash-flow tuffs gain further insights into the development of syn- G. A. Smith, 1994, personal commun.) are typi- with sources to the west. volcanic extensional basins, we briefly review and cal of volcanic-dominated rifts (Table 2). The Field observations, new magnetic polarity summarize key features of several intra-arc rifts Eocene Panther Creek half graben and the data, and 40Ar/39Ar age determinations show that (Table 2). Intra-arc extensional basins vary in form, nearby Custer graben (McIntyre et al., 1982; the tuffs of Challis Creek and tuffs of Castle the nature of the basin-fill sequences, and relative McIntyre and Johnson, 1985; Allen and Moye, Rock may be the same unit, as originally sug- and absolute rates of extension and magmatism 1990; Allen and Hahn, 1994) are also volcanic- gested by McIntyre et al. (1982). This tentative (Table 2) (see also Smith and Landis, 1995). dominated rifts. The Republic graben of north- correlation allows us to (1) calculate the peak Many basins contain large proportions of sed- east Washington shows that more than one type rate of extension in the Panther Creek half gra- imentary rocks, much of it reworked from vol- of rift may characterize an individual volcanic ben, (2) expand the lateral distribution of the canic sources (Table 2). The Summit basins of arc. The Republic graben formed in the same tuffs of Challis Creek by at least 700 km2, and the Aleutian chain have 4 km of fine-grained vol- Eocene arc as the Panther Creek and Custer gra- (3) simplify the stratigraphy of the Challis Vol- caniclastic sediments (Scholl et al., 1975), and bens, yet the proportion of sedimentary rocks in canic group. The Castle Rock trap-door caldera the grabens that formed in the Japanese arc have the Republic graben far exceeds that in the Pan- of Ekren (1985), the inferred source of the tuffs as much as 3 km of sediment deposited during ther Creek or Custer grabens (Table 2) (Carlson of Castle Rock, may be a synvolcanic half gra- and after a rapid rifting episode (Yamaji, 1990); and Moye, 1990; Meussig, 1962, 1967; Staatz, ben similar to the Panther Creek half graben, both are typical of sediment-dominated rifts 1964; Pearson and Obradovich, 1977; Suydam, rather than a volcano-tectonic depression. (Table 2). Clast composition data from the Pan- 1993; Mathews and Gaylord, 1994; D. Gaylord, The geometry of the basin-fill deposits shows ther Creek half graben demonstrate, however, 1994, personal commun.). that movement on the two nearly perpendicular that volcaniclastic rocks are not necessarily the The structure of intra-arc rifts is also variable; basin-bounding normal faults was synchronous dominant sediment type in intra-arc rifts, and that ranging from full to half grabens oriented paral- and that both faults controlled the development of the absence of volcaniclastic sedimentation does lel or perpendicular to the arc (Table 2). Full the Panther Creek half graben. The two basin- not preclude the interpretation of an intra-arc rift grabens appear to be somewhat more common bounding faults may have different attitudes be- setting (see also Riggs and Busby-Spera, 1990). than half grabens and most rifts parallel the arc. cause they reactivated preexisting(?) northeast- Some extensional basins within volcanic arcs The Mexican Volcanic belt is a noteworthy ex- and northwest-striking strike-slip faults. We found contain dominantly volcanic rocks. The High ception because coeval rift basins developed

Geological Society of America Bulletin, March 1997 265

TABLE A1. PALEOMAGNETIC DATA FROM SITES IN THE PANTHER CREEK HALF GRABEN AND OUTFLOW OF THE TUFFS OF CHALLIS CREEK, IDAHO α Site Rock unit N/No Geographic Tilt corr. k 95 R P Latitude Longitude no. Decl. Incl. Decl. Incl. (¡N) (¡W) 1 Tuffs of Castle Rock, densely welded 7/7 273.3 77.7 107.0 88.3 473.5 2.8 6.9873 N 44¡56′58″ 114¡20′42″ 2 Tuffs of Castle Rock, nonwelded 6/6 11.3 66.4 4.7 52.9 29.5 12.5 5.8306 N 44¡57′17″ 114¡20′49″ 3 Fractured rhyolite tuff of Moyer Ridge 7/7 334.7 65.9 334.7 65.9 187.1 4.4 6.9679 N 44¡58′41″ 114¡18′16″ 6 Outflow of the tuffs of Challis Creek 8/8 314.2 64.2 356.4 72.9 1192.7 1.6 7.9941 N 44¡30′28″ 114¡14′20″

Note: N/No, ratio of the number of samples used in calculating site mean direction to number of samples analyzed; Decl.—declination; α Incl.—inclination; k—best estimate of Fisher precision parameter; 95, half-angle of cone of 95% confidence; R, length of resultant vector; P, polarity.

both parallel and perpendicular to the arc (Allan, tivity, may aid in the interpretation of rifts in the Great Basin of the western United States: Geological Society of America Bulletin, v. 105, p. 56Ð76. 1986; Righter et al., 1995). Angular unconformi- geologic record. The Panther Creek half graben Bartels, E., Douglas, I., Van Huffel, G., and Busby, S., 1990, Tobacco ties are found in most rifts, particularly those is an unambiguous example of a rift basin that Root Geological Society tour of the Beartrack Project Mackinaw mining district, Lemhi county, Idaho, in Moye, F. J., ed., Geology with a half-graben geometry (Table 2). formed extremely rapidly during an episode of and ore deposits of the Trans-Challis fault system/Great Falls tec- Basin fill accumulated at rates that vary by an voluminous volcanism, and could serve as a tonic zone: Missoula, Montana, Tobacco Root Geological Soci- ety, p. 31Ð36. order of magnitude, from about 0.2 mm/yr in the model for other such synvolcanic rifts. Bennett, E. H., 1977, Reconnaissance geology and geochemistry of the Mexican Volcanic belt to about 3 mm/yr in the Blackbird MountainÐPanther Creek region, Lemhi county, Idaho: Idaho Bureau of Mines and Geology Pamphlet 167, 108 p. Miocene Japanese arc and the Panther Creek ACKNOWLEDGMENTS Bennett, E. H., 1986, Relationship of the Trans-Challis fault system in central Idaho to Eocene and Basin and Range extension: Geology, half graben. Comparison of the Japan arc and the v. 14, p. 481Ð484. Panther Creek half graben illustrates that the ab- This work was partially funded by a Utah Bennett, E. H., and Knowles, C. R., 1985, Tertiary plutons and related rocks in central Idaho, in McIntyre, D. H., ed., Symposium on the solute rate of subsidence does not determine State University Faculty Grant and National Sci- geology and mineral deposits of the Challis 1¡ × 2¡ quadrangle, whether a basin fills mostly with volcanic or sed- ence Foundation grant EAR-9317395 to Jan- Idaho: U.S. Geological Survey Bulletin 1658-C, p. 81Ð95. Best, M. G., and Christiansen, E. H., 1991, Limited extension during imentary rocks. Both basins subsided rapidly, ecke. We appreciate the prompt and helpful re- peak Tertiary volcanism, Great Basin of Nevada and Utah: Jour- yet the Panther Creek half graben filled with vol- view of the manuscript by Nancy Riggs. We nal of Geophysical Research, v. 96, p. 13509Ð13528. Bond, J. G., 1978, Geologic map of Idaho: Idaho Bureau of Mines and canic rocks, whereas the Japanese rifts filled thank Cheryl Brown for help in interpreting the Geology, scale 1:500 000. Busby-Spera, C., 1988, Speculative tectonic model for the early Meso- mostly with sediment during the most rapid conglomerate and tuffaceous sediments, Gary A. zoic arc of the southwest Cordilleran United States: Geology, phases of subsidence. We infer that the relative Smith and David R. Gaylord for providing un- v. 16, p. 1121Ð1125. Carlson, D. H., and Moye, F. J., 1990, The Colville igneous complex: rate of volcanism and subsidence is the principal published data for Table 2, and James P. Evans Paleogene volcanism, plutonism and extension in northeastern influence on the proportion of volcanic rocks in for suggestions. A review by E. B. Ekren of an Washington, in Anderson, J. L., ed., The nature and origin of Cor- dilleran magmatism: Geological Society of America Memoir 174, the basin fill (Fig. 8). Sediments dominate in earlier version and discussions with R. F. Hardy- p. 375Ð394. those intra-arc rift basins where volcanism does man about the relationship between the tuffs of Cole, J. W., 1979, Structure, petrology, and genesis of Cenozoic volcanism, Taupo volcanic zone, New Zealand—A review: New not continually fill the rift with volcanic rocks. Castle Rock and tuffs of Challis Creek are ap- Zealand Journal of Geology and Geophysics, v. 22, p. 631Ð657. Dalrymple, G. B., Alexander, E. C. J., Lanphere, M. A., and Kraker, G. Studies in the Great Basin suggest that rapidly preciated. Douglas Wilder helped collect the pa- P., 1981, Irradiation of samples for 40Ar/39Ar dating using the subsiding synvolcanic rift basins contain abundant leomagnetic samples. Stereonet version 4.6 by Geological Survey TRIGA reactor: U.S. Geological Survey Pro- fessional Paper 1176, p. 55. volcanic rocks and thick volcaniclastic deposits, Richard Allmendinger facilitated data analysis Davison, I., 1986, Listric normal fault profiles: Calculation using bed- and display well-developed angular unconformi- and presentation. length balance and fault displacement: Journal of Structural Ge- ology, v. 8, p. 209Ð210. ties (Gans et al., 1989). Rarely are most of these Desmarais, N. R., 1983, Geology and geochronology of the Chief features well developed in intra-arc rift basins APPENDIX Joseph plutonic- metamorphic complex, Idaho-Montana [Ph.D. dissert.]: Seattle, University of Washington, 143 p. (Table 2). Rather, volcanic rocks may not compose Paleomagnetic data from sites in the Panther Creek Dula, W. F., Jr., 1991, Geometric models of listric normal faults and half graben and outflow of the tuffs of Challis Creek, rollover folds: American Association of Petroleum Geologists a major part of the basin fill, the sedimentary rocks Bulletin, v. 75, p. 1609Ð1625. may be derived mostly from nonvolcanic source Idaho, are shown in Table A1. Ekren, E. B., 1981, Van Horn Peak—A welded tuff vent in central Idaho, in Montana Geological Society 1981 Field Conference areas, conformable sequences characterize some REFERENCES CITED Southwest Montana: Billings, Montana, p. 311Ð316. synvolcanic rifts, and rates of extension may be Ekren, E. B., 1985, Eocene cauldron-related volcanic events in the Chal- Allan, J. F., 1986, Geology of the northern Colima and Zacoalco lis quadrangle, in McIntyre, D. H., ed., Symposium on the geol- low (<0.5 mm/yr). 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266 Geological Society of America Bulletin, March 1997

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