Madison Range Research Articles

Is Habitat Constraining Bighorn Sheep Restoration? A Case Study

ABSTRACTRocky Mountain bighorn sheep (Ovis canadensis) restoration continues to be a challenge throughout western North America despite nearly a century of efforts dedicated to the species' recovery. Though bighorn sheep restoration may be constrained by several environmental factors and behavioral tendencies, areas with unrealized restoration potential may exist if novel restoration strategies are considered. We used global positioning system (GPS) location data from 27 female bighorn sheep within the southern portion of the Madison Range in southwest Montana, USA, 2015–2017, to develop and validate winter and summer habitat models, which we extrapolated throughout the entire Madison Range to identify potential seasonal habitat. We estimated potential bighorn sheep minimum population estimates within the extrapolation area by linking our top‐ranked winter habitat model to population count data. During summer, female bighorn sheep selected areas characterized by rugged and steep terrain, reduced canopy cover, southwestern aspects, and ridgelines. During winter, female bighorn sheep selected areas characterized by low elevations, southwestern aspects, steep slopes, reduced canopy cover, ridgelines, high normalized difference vegetation index amplitude, and areas close to steep terrain. Predicted summer habitat was concentrated along the high‐elevation ridgelines associated with steep slopes and reduced canopy cover. Predicted winter habitat occurred in a non‐contiguous distribution primarily along the low‐elevation, southwest‐facing aspects along the western slopes of the Madison Range. Our results suggest that the Madison Range may be capable of supporting 780–1,730 animals, which is 2–4 times the number of bighorn sheep currently observed within the range. Further, our findings provide managers with a quantification of female bighorn sheep habitat and suggests that a strategy focused on establishing a metapopulation through a series of within‐range translocations may enhance bighorn sheep restoration. We suggest that similar restoration opportunities may be common in other unoccupied areas of bighorn sheep historical range. © 2020 The Wildlife Society.

Early Inception of the Laramide Orogeny in Southwestern Montana and Northern Wyoming: Implications for Models of Flat‐Slab Subduction

AbstractTiming and distribution of magmatism, deformation, exhumation, and basin development have been used to reconstruct the history of Laramide flat‐slab subduction under North America during Late Cretaceous‐early Cenozoic time. Existing geodynamic models, however, ignore a large (~40,000‐km2) sector of the Laramide foreland in southwestern Montana. The Montana Laramide ranges consist of Archean basement arches (fault‐propagation folds) that were elevated by thrust and reverse faults. We present new thermochronological and geochronological data from six Laramide ranges in southwestern Montana (the Beartooth, Gravelly, Ruby and Madison Ranges, and the Tobacco Root and Highland Mountains) that show significant cooling and exhumation during the Early to mid‐Cretaceous, much earlier than the record of Laramide exhumation in Wyoming. These data suggest that Laramide‐style deformation‐driven exhumation slightly predates the eastward sweep of magmatism in western Montana, consistent with geodynamic models involving initial strain propagation into North American cratonic rocks due to stresses associated with a northeastward expanding region of flat‐slab subduction. Our results also indicate various degrees of Cenozoic heating and cooling possibly associated with westward rollback of the subducting Farallon slab, followed by Basin‐and‐Range extension.

Dating Metasomatism: Monazite and Zircon Growth during Amphibolite Facies Albitization

We present coupled textural observations and trace element and geochronological data from metasomatic monazite and zircon, to constrain the timing of high-grade Na-metasomatism (albitization) of an Archean orthogneiss in southwest Montana, USA. Field, mineral textures, and geochemical evidence indicate albitization occurred as a rind along the margin of a ~3.2 Ga granodioritic orthogneiss (Pl + Hbl + Kfs + Qz + Bt + Zrn) exposed in the Northern Madison range. The metasomatic product is a weakly deformed albitite (Ab + Bt + OAm + Zrn + Mnz + Ap + Rt). Orthoamphibole and biotite grew synkinematically with the regional foliation fabric, which developed during metamorphism that locally peaked at upper amphibolite-facies during the 1800–1710 Ma Big Sky orogeny. Metasomatism resulted in an increase in Na, a decrease in Ca, K, Ba, Fe, and Sr, a complete transformation of plagioclase and K-feldspar into albite, and loss of quartz. In situ geochronology on zoned monazite and zircon indicate growth by dissolution–precipitation in both phases at ~1750–1735 Ma. Trace element geochemistry of rim domains in these phases are best explained by dissolution–reprecipitation in equilibrium with Na-rich fluid. Together, these data temporally and mechanistically link metasomatism with high-grade tectonism and prograde metamorphism during the Big Sky orogeny.

Foreland-directed propagation of high-grade tectonism in the deep roots of a Paleoproterozoic collisional orogen, SW Montana, USA

The study of deeply exhumed ancient collisional belts offers important constraints on geologic processes and properties complementary to inaccessible portions of the crustal column in active orogens. The ca. 1.8−1.7 Ga Big Sky orogeny in southwest Montana is a major convergent belt associated with the Proterozoic amalgamation of Laurentia. New structural, petrologic, and geochronologic data from the Northern Madison Range, crossing the NE-SW trend of the belt, record key information about the internal dynamics of the orogen. At least two phases of Big Sky−related deformation are preserved, both nearly coeval with peak metamorphic conditions of ∼0.9−0.8 GPa and >700 °C. Metamorphic zircon grains from a deformed mafic dike yield a weighted mean ion probe U-Pb date of 1737 ± 28 Ma (2σ). Monazite grains from a metapelite yield electron microprobe U-Th total-Pb dates of ca. 1750−1705 Ma, spanning prograde, peak, and retrograde intervals. Exposed Proterozoic paleodepths range from deeper levels (∼45−40 km; 1.2 GPa) in the northwestern end of the range to shallower levels (∼30−25 km) in the central-southeast area. The age of high-grade tectonism appears to become younger southeastward away from the core of the orogen, from ca. 1810−1780 Ma in the Highland Mountains, to ca. 1780−1750 Ma in the Ruby Range, Tobacco Root Mountains, and northwesternmost Northern Madison Range, and 1750−1720 Ma in the central Northern Madison Range. These spatial and temporal patterns of lateral growth and propagation of the orogen are similar to those observed in other collisional orogenic systems, and they may reflect multiple collision phases, protracted collision, and/or postcollisional collapse.

Strain localization in the Spanish Creek mylonite, Northern Madison Range, southwest Montana, U.S.A.

The Spanish Creek mylonite zone formed within multiply deformed granitic orthogneisses in the Northern Madison Range in southwestern Montana. An integrated approach incorporating fieldwork, microstructural analysis, electron backscatter diffraction (EBSD), and U-Pb zircon geochronology was utilized to characterize this mylonite zone and place its evolution into a broader regional context. The mylonitic rocks are characterized by a well-developed L-S fabric and a variably porphyroclastic character. U-Pb zircon geochronology yielded a magmatic crystallization age of 2824 ± 47 Ma (2σ) for the granitic protolith of the porphyroclastic Spanish Creek mylonite, indicating that the mylonitic deformation could have occurred during either a late Archean tectonic event or the late Paleoproterozoic Big Sky Orogeny, potentially activated and then reactivated with each successive event. Three distinct structural realms defined within the study area on the basis of structural observations and EBSD data reveal differences in the degree of mylonitic deformation. Two models are herein proposed to explain these significant differences, which are manifest in deformation microstructures and CPO fabrics between structural realms. In the first model, the intrusion and mylonitization of the granitic protolith for the rocks of the third structural realm were broadly coeval during a late Archean tectonic event, allowing for high-temperature mylonitic deformation. In an alternate model, the mylonitic deformation significantly postdated the ∼2.8 Ga intrusion of the granitic protolith, with factors aside from temperature influencing the nature of the deformation. Strain localization provides a viable explanation for the observed differences in the intensity of deformation between structural realms.

Quartz shielding of sub-10 μm zircons from radiation damage-enhanced Pb loss: An example from a metamorphosed mafic dike, northwestern Wyoming craton

The coupling of two new approaches enabled acquisition of in situ U–Pb data from zircons as small as 5μm in a metamorphosed mafic dike from the Northern Madison Range, Montana. Despite negligible zircon yield by mineral separation, automated mineralogy rapidly identified >375 sub-20μm zircons in a single thin section. Subsequently, zircon crystals were dated using modifications to the conventional SIMS technique to preferentially collect secondary ions emitted from a domain a few microns in size within the ∼20μm diameter analysis pit. This approach allowed analysis of zircons too small to be dated by standard SIMS and TIMS methods. U–Pb data define a discordia array with upper and lower intercepts of 1753±9Ma and 63±8Ma, respectively (1σ error, MSWD=1.5). We interpret the upper intercept to reflect zircon growth during high-temperature and high-pressure metamorphism (800°C, 1.2GPa) based on textural relationships between the dated zircons and the peak metamorphic assemblage. The lower intercept is attributable to the thermal pulse associated with emplacement of the nearby ca. 75Ma Tobacco Root batholith. Percent discordance is linked with both textural setting and U concentration. Zircons located along grain boundaries or within fractured host grains display a positive correlation between U (1049–2817ppm) and percent discordance (12–82%) that is consistent with radiation damage-enhanced Pb loss. In contrast, three zircons housed completely within unfractured quartz yield the most concordant analyses of the dataset (≤7% discordant), despite U concentrations comparable to highly discordant matrix grains. This relationship suggests that the included zircons were shielded from Pb loss by the encapsulating quartz crystals. The results imply that targeting zircons located completely within unfractured host phases may aid in isolating earlier portions of geologic histories. The primary geological implication of this dataset is to increase the documented extent of Paleoproterozoic high-grade metamorphism in the northwestern Wyoming craton.

Structural development of high-temperature mylonites in the Archean Wyoming province, northwestern Madison Range, Montana

The Crooked Creek mylonite, in the northwestern Madison Range, southwestern Montana, is defined by several curved lenses of high non-coaxial strain exposed over a 7-km-wide, northeast-trending strip. The country rocks, part of the Archean Wyoming province, are dominantly trondhjemitic to granitic orthogneiss with subordinate amphibolite, quartzite, aluminous gneiss, and sills of metabasite (mafic granulite). Data presented here support an interpretation that the mylonite formed during a period of rapid, heterogeneous strain at near-peak metamorphic conditions during an early deformational event (D1) caused by northwest–southeast-directed transpression. The mylonite has a well-developed L-S tectonite fabric and a fine-grained, recrystallized (granoblastic) texture. The strong linear fabric, interpreted as the stretching direction, is defined by elongate compositional “fish,” fold axes, aligned elongate minerals, and mullion axes. The margins of the mylonitic zones are concordant with and grade into regions of unmylonitized gneiss. A second deformational event (D2) has folded the mylonite surface to produce meter- to kilometer-scale, tight-to-isoclinal, gently plunging folds in both the mylonite and country rock, and represents a northwest–southeast shortening event. Planar or linear fabrics associated with D2 are remarkably absent. A third regional deformational event (D3) produced open, kilometer-scale folds generally with gently north-plunging fold axes. Thermobarometric measurements presented here indicate that metamorphic conditions during D1 were the same in both the mylonite and the country gneiss, reaching upper amphibolite- to lower granulite-facies conditions: 700 ± 50° C and 8.5 ± 0.5 kb. Previous geochronological studies of mylonitic and cross-cutting rocks in the Jerome Rock Lake area, east of the Crooked Creek mylonite, bracket the timing of this high-grade metamorphism and mylonitization between 2.78 and 2.56 Ga, nearly a billion years before the 1.78-Ga Big Sky orogeny, which overprinted the basement rocks exposed in adjacent ranges of the Wyoming province.

Coyotes and recolonizing wolves: social rank mediates risk-conditional behaviour at ungulate carcasses

Wolf, Canis lupus, recolonization of the Greater Yellowstone Ecosystem provides a rare opportunity to identify behaviours facilitating coexistence between sympatric canids. We investigated interactions between coyotes, Canis latrans, and recolonizing wolves at ungulate carcasses in Montana's Madison Range. We used a field-experimental study design consisting of a two-level carcass treatment (wolf presence, wolf absence) to assess factors influencing coyote risk assessment, carrion consumption and aggressive encounters with wolves. Socially dominant coyotes (alphas and betas) responded to wolf presence by increasing the proportion of time spent vigilant while scavenging. Vigilance behaviour was more pronounced when scavenging closer to structurally complex vegetation where lateral occlusion inhibited the ability of coyotes to scan for, and possibly escape from, returning wolves. Despite greater time spent vigilant, alpha coyotes consumed the greatest amount of carrion biomass by feeding on carcasses in earlier stages of consumption when organs and large muscle tissues were still present. This finding suggests that alpha coyotes might trade off greater risk for higher-quality food items. Coyotes would aggressively confront wolves: numerical advantage by coyotes and the stage of carcass consumption were influential in determining whether coyotes were able to displace wolves from carcasses. Coyotes relied on a gradient of risk-sensitive behaviours, ranging from elevated vigilance to aggressive confrontation, to manage risk associated with wolf presence. Identification of these behaviours, and their sensitivity to numeric and social factors, is an important step in elucidating mechanisms of resource partitioning in social canids.

Stratigraphy and structural setting of Upper Cretaceous Frontier Formation, western Centennial Mountains, southwestern Montana and southeastern Idaho

Stratigraphic, sedimentologic, and palynologic data were used to correlate the Frontier Formation of the western Centennial Mountains with time-equivalent rocks in the Lima Peaks area and other nearby areas in southwestern Montana. The stratigraphic interval studied is in the middle and upper parts (but not uppermost) of the formation based on a comparison of sandstone petrography, palynologic age data, and our interpretation of the structure using a seismic line along the frontal zone of the Centennial Mountains and the adjacent Centennial Valley. The Frontier Formation is comprised of sandstone, siltstone, mudstone, limestone, and silty shale in fluvial and coastal depositional settings. A distinctive characteristic of these strata in the western Centennial Mountains is the absence of conglomerate and conglomeratic sandstone beds. Absence of conglomerate beds may be due to lateral facies changes associated with fluvial systems, a distal fining of grain size, and the absence of both uppermost and lower Frontier rocks in the study area. Palynostratigraphic data indicate a Coniacian age for the Frontier Formation in the western Centennial Mountains. These data are supported by a geochronologic age from the middle part of the Frontier at Lima Peaks indicating a possible late Coniacian-early Santonian age (86.25 ± 0.38 Ma) for the middle Frontier there. The Frontier Formation in the western Centennial Mountains is comparable in age and thickness to part of the Frontier at Lima Peaks. These rocks represent one of the thickest known sequences of Frontier strata in the Rocky Mountain region. Deposition was from about 95 to 86 Ma (middle Cenomanian to at least early Santonian), during which time, shoreface sandstone of the Telegraph Creek Formation and marine shale of the Cody Shale were deposited to the east in the area now occupied by the Madison Range in southwestern Montana. Frontier strata in the western Centennial Mountains are structurally isolated from other Cretaceous rocks in the region and are part of the Lima thrust sheet that lies at the leading edge of the Sevier-style overthrusting in this part of southwestern Montana and adjacent southeastern Idaho.

Comparative Patterns of Predation by Cougars and Recolonizing Wolves in Montana's Madison Range

Abstract: Numerous studies have documented how prey may use antipredator strategies to reduce the risk of predation from a single predator. However, when a recolonizing predator enters an already complex predator—prey system, specific antipredator behaviors may conflict and avoidance of one predator may enhance vulnerability to another. We studied the patterns of prey selection by recolonizing wolves (Canis lupus) and cougars (Puma concolor) in response to prey resource selection in the northern Madison Range, Montana, USA. Elk (Cervus elaphus) were the primary prey for wolves, and mule deer (Odocoileus hemionus) were the primary prey for cougars, but elk made up an increasingly greater proportion of cougar kills annually. Although both predators preyed disproportionately on male elk, wolves were most likely to prey on males in poor physical condition. Although we found that the predators partitioned hunting habitats, structural complexity at wolf kill sites increased over time, whereas complexity of cougar kill sites decreased. We concluded that shifts by prey to structurally complex refugia were attempts by formerly naïve prey to lessen predation risk from wolves; nevertheless, shifting to more structurally complex refugia might have made prey more vulnerable to cougars. After a change in predator exposure, use of refugia may represent a compromise to minimize overall risk. As agencies formulate management strategies relative to wolf recolonization, the potential for interactive predation effects (i.e., facilitation or antagonism) should be considered.

New 40Ar/39Ar age determinations and paleomagnetic results bearing on the tectonic and magmatic history of the northern Madison Range and Madison Valley region, southwestern Montana, U.S.A.

Detailed 40 Ar/ 39 Ar dating and paleomagnetic analysis of dacite porphyry sills and dikes that intrude Cretaceous sedimentary rocks in the northern Madison Range in southwestern Montana show that Laramide shortening was essentially complete by ∼69 Ma. A negative paleomagnetic fold test indicates that Laramide folding occurred before cooling of the dacite sills and dikes at ∼69 Ma. Laramide deformation began synchronous with deposition of the Livingston Formation rocks at ∼79 Ma. These results are consistent with previous observations in the region that show the onset of Laramide deformation in the northern Rocky Mountains becoming progressively younger toward the east. 40 Ar/ 39 Ar dating of additional igneous rocks in the northern Madison Valley and around Norris, Montana better define post-Laramide tectonomagmatic events in the region, including Eocene–Oligocene volcanism and Basin and Range crustal extension. Dates from three rhyolitic intrusions near Red Mountain are between 48.71 ± 0.18 Ma and 49.42 ± 0.18 Ma, similar to the dates from basal silicic flows of the Virginia City volcanic field (part of the southwest Montana volcanic province), suggesting that the Red Mountain intrusions may have been the sources for some of the early extrusive rocks. Magmatism in the Virginia City volcanic field became generally more mafic with time, and a ∼30-Ma basalt flow near Norris is considered a late, outlying member of the volcanic field. A tuff along the east side of the Madison Valley half graben yielded a early middle Miocene date (16.2 ± 0.19 Ma), suggesting that accelerated crustal extension and associated rapid basin sedimentation probably began in the early Miocene, slightly earlier than previous estimates.

Variations in snow strength and stability on uniform slopes

This research investigated whether single snowpits could reliably represent snowpack strength and stability conditions throughout apparently ‘uniform’ slopes. Seven slopes were selected by experienced avalanche forecasters, three each in the Bridger and Madison Ranges of Southwest Montana (USA), and one in the Columbia Mountains near Rogers Pass, British Columbia (Canada). Teams performed 10 quantified loaded column tests in each of five snowpits within a 900 m 2 sampling site on each ‘uniform’ slope, measuring shear strength in a single weak layer. Collection of slab shear stress data enabled the calculation of a stability index, S QLCT. Altogether, eleven trials were performed during 2000/2001 and 2001/2002, testing several weak layer types exhibiting a wide range of strengths. Weak layer strength and slab stress conditions varied widely across the sampling sites, with coefficients of variation in weak layer shear strength ranging from 10% to 50%, coefficients of variation in shear stress from 2% to 48%, and stability indices ranging from 1.8 to 5.7. Of the 54 snowpits completed, 10 pits were empirically rejected as unrepresentative of the stability index at their sampling sites. Of the remaining 44 statistically analyzed pits, 33 pits were statistically representative of their site-wide stability index, and the other 11 pits were found statistically unrepresentative of their site. All five snowpits at a site were statistically representative of their site-wide stability index in three of the eleven trials. The frequent inability of single pits to reliably represent stability on those eleven 900 m 2 sampling sites, located on apparently ‘uniform’ slopes, highlights the importance of improving our understanding of the processes affecting the variability of snowpack stability on any given day and the uncertainties associated with ‘point’ stability data.

Correlation of Upper Cretaceous strata from Lima Peaks area to Madison Range, southwestern Montana and southeastern Idaho, USA

An 40Ar/ 39Ar age of 85.81 Ma±0.22 my was obtained on sanidine from a volcanic procellanite bed near the top of the 2135+m-thick Upper Cretaceous Frontier Formation in the Lima Peaks area of southwestern Montana. This early Santonian age, combined with previously determined age data including a palynological age of Cenomanian for the lower Frontier at Lima Peaks, and a U-Pb isotopic date of about 95 Ma for the base of the Frontier Formation in the eastern Pioneer Mountains north of the Lima Peaks area, provides an age range for the nonmarine formation. In the Madison Range, farther east in southweastern Montana, this age range corresponds to marine strata of not only the Frontier Formation, but also the overlying Cody Shale and Telegraph Creek Formation, a sequence that totals less than 760 m thick. The Upper Cretaceous marine formations of the madison Range are closely zoned by molluscan faunas that are well constrained with radiometric dates. The 40Ar/ 39Ar age of 85.81 Ma±0.22 my at Lima Peaks is bracketed by radiometric dates for the Scaphites depressus—Protexanites bourgeoisianusbiozone and the overlying Clioscaphites saxitonianus—Inoceramus undulatopilcatusbiozone of the Western Interior. Fossils of both of these biozones are present in the Cody Shale and the Telegraph Creek Formation in the Madison Range. The Telegraph Creek contains two units of volcanic ash that are approximate time equivalents of the volcanic procellanite of the Lima Peaks area. Clasts in the conglomerate of the upper part of the Frontier in the Lima Peaks area were shed during the initial stages of uplift of the Blacktail-Snowcrest Highlands which rose to the north. The dated porcellanite lies above the conglomerates and indicates that the uplift was initiated by middle or late Coniacian, 87–88 Ma.

Source Rock Potential of Middle Cretaceous Rocks in Southwestern Montana

The middle Cretaceous in southwestern Montana is composed of a marine and nonmarine succession of predominantly clastic rocks that were deposited along the western margin of the Western Interior Seaway. In places, middle Cretaceous rocks contain appreciable total organic carbon (TOC), such as 5.59% for the Mowry Shale and 8.11% for the Frontier Formation in the Madison Range. Most samples, however, exhibit less than 1.0% TOC. The genetic or hydrocarbon potential (S1+S2) of all the samples analyzed, except one, yield less than 1 mg HC/g rock, strongly indicating poor potential for generating commercial amounts of hydrocarbons. Out of 51 samples analyzed, only one (a Thermopolis Shale sample from the Snowcrest Range) showed a moderate petroleum potenti l of 3.1 mg HC/g rock. Most of the middle Cretaceous samples are thermally immature to marginally mature, with vitrinite reflectance ranging from about 0.4 to 0.6% R0. Maturity is high in the Pioneer Mountains, where vitrinite reflectance averages 3.4% R0, and at Big Sky, Montana, where vitrinite reflectance averages 2.5% R0. At both localities, high R0 values are due to local heat sources, such as the Pioneer batholith in the Pioneer Mountains.

Basement and Cover-Rock Deformation During Laramide Contraction in the Northern Madison Range (Montana) and Its Influence on Cenozoic Basin Formation

Two major Laramide fault systems converge in the northwestern Madison Range: the northwest-striking, southwest-vergent Spanish Peaks reverse fault and the north-striking, east-vergent Hilgard thrust system. Analysis of foliation attitudes in basement gneiss north and south of the Spanish Peaks fault indicates that the basement in thrusted blocks of the Hilgard thrust system has been rotated by an amount similar to that of the basement-cover contact. Steeply dipping, north-striking breccia zones enclosing domains of relatively undeformed basement may have permitted domino-style rotation of basement blocks during simple shear between pairs of thrusts. In most places along the Hilgard thrust system, a large basement overhang, produced by thrusting of Archean blocks above rocks as young as Late Cretaceous, overlies a tight footwall syncline. This tight folding is largely concentric and was accommodated by flexural slip, resulting in severe crowding in synclinal hinges that resulted in observed or inferred features such as bedding-plane slip, imbricate and out-of-syncline thrusting, and hinge collapse. The north-striking Madison normal fault system, a zone of Tertiary and Quaternary valley-forming normal faults, is approximately parallel to the Hilgard thrust system. In some places, normal faults are reactivated thrusts on which large basement overhangs of the Hilgard thrust system were dropped back into the Madison Valley and covered by Tertiary basin-fill deposits, leaving only the rocks of the footwall synclines exposed. In other places, both the thrusted Archean blocks and the near-isoclinal footwall synclines are well preserved. This paired fault system (the Madison normal fault system and the Hilgard thrust system) of the northern Madison Range is strikingly similar to other paired systems in southwestern Montana along and adjacent to the western margins of the Ruby Range, Snowcrest Range, Greenhorn Range, Tobacco Root Mountains, and Bridger Range. Such systems may be the result of collapse of the crestal zones of large Laramide basement uplifts (arches) during Tertiary extension. No hydrocarbon discoveries have been made in this unique structural province. However, petroleum exploration here has focused on basement-cored anticlines, both surface and subthrust, related to the two major Laramide fault systems and on the fault-bounded blocks of Tertiary rocks within the post-Laramide extensional basins. The interplay of the two Laramide fault systems during both Laramide shortening and Tertiary extension has produced a variety of possible structural traps in the Madison Range that have not yet been thoroughly investigated.

Reconciling the roles of tectonism and climate in Quaternary alluvial fan evolution

Tectonism and climate are the primary variables considered in conceptual models of alluvial fan evolution and basin-fill architecture, yet their roles and relative importance are far from resolved. The Madison Range and Madison River valley, southwestern Montana, were affected during late Quaternary time by varying degrees of tectonic activity as well as by climate change through multiple glaciations; we are thus able to evaluate the impact of tectonism and climate on fan evolution. Two primary fan deposits were correlated along the range front. They were deposited as proglacial outwash fans during glacial periods of increased sediment and water discharge. During the present interglacial period, these deposits were entrenched and secondary fans formed on lower fan areas. Temporal and stratigraphic relations indicate that climate was largely responsible for driving both fan aggradation and entrenchment. In contrast, stratigraphic relations between fan deposits and surface faults do not support a direct relation, either spatial or temporal, between local faulting and fan deposition. Because tectonism produces and maintains the relief necessary for fans to form, its primary role is long term, controlling the duration over which fan deposition may occur along a mountain front. If Quaternary alluvial fan evolution is representative of ancient alluvial fan evolution, this study has important implications for interpreting primary controls on deposition of the ancient alluvial fan sequences.

Age and composition of Archean crystalline rocks from the southern Madison Range, Montana: Implications for crustal evolution in the Wyoming craton

The southern Madison Range of southwestern Montana contains two distinct Precambrian lithologic assemblages: (1) a complex of tonalitic to granitic gneisses that has been thrust over (2) a medium-grade metasupracrustal sequence dominated by pelitic schist. Crystallization ages for the protolith of a granodioritic gneiss that intruded the metasupracrustal sequence ([approximately]2.6 Ga)-along with an intercalated meta-andesite ([approximately]2.7 Ga) confirm the sequence as Archean. Chemical (major and trace element), isotopic (Rb-Sr, Sm-Nd, Pb-Pb), and geochronologic (U-Pb zircon) data for selected components of the gneiss complex indicate two groups of gneisses: an older, tonalitic to trondhjemitic group ([approximately]3.3 Ga) and a younger, mostly granitic group ([approximately]2.7 Ga). Both groups of gneisses exhibit the radiogenic Pb and nonradiogenic Nd isotopic signature characteristic of Middle and Late Archean rocks from throughout the Wyoming province. The older gneisses, in particular, appear to be compositionally, isotopically, and chronologically comparable to other Middle Archean gneisses from the northern part of the province (for example, Beartooth Mountains). The Late Archean gneisses, however, exhibit some distinct differences relative to their temporal counterparts, including (1) trace-element patterns that are more suggestive of crustal melts than subduction activity and (2) higher initial Sr isotopic ratios that suggest more involvement of older crustmore » in their petrogenesis. These comparisons suggest that the juxtaposition of Late Archean terranes in the northern Wyoming province was the result, at least in part, of intracratonic processes. 41 refs., 6 figs., 2 tabs.« less

The nature of Archean terrane boundaries: an example from the northern Wyoming Province

The Archean northern Wyoming Province can be subdivided into two geologically distinct terranes, the Beartooth-Bighorn magmatic terrane (BBMT) and the Montana metasedimentary terrane (MMT). The BBMT is characterized by voluminous Late Archean (2.90-2.74 Ga) magmatic rocks (primarily tonalite, trondhjemite, and granite); metasedimentary rocks are preserved only as small, rare enclaves in this magmatic terrane. The magmatic rocks typically have geochemical and isotopic signatures that suggest petrogenesis in a continental magmatic arc environment. The MMT, as exposed in the northern Gallatin and Madison Ranges, is dominated by Middle Archean trondhjemitic gneisses (3.2–3.0 Ga); metasedimentary rocks, however, are significantly more abundant than in the BBMT. Each terrane has experienced a separate and distinct geologic history since at least 3.6 Ga ago based on differences in metamorphic and structural styles, composition of magmatic and metasupracrustal rocks, and isotopic ages; consequently, these may be described as discrete terranes in the Cordilleran sense. Nonetheless, highly radiogenic and distinctive Pb-Pb isotopic signatures in rocks of all ages in both terranes indicate that the two terranes share a significant aspect of their history. This suggests that these two Early to Middle Archean crustal blocks, that initially evolved as part of a larger crustal province, experienced different geologic histories from at least 3.6 Ga until their juxtaposition in the Late Archean (between 2.75 to 2.55 Ga ago). Consequently, the boundary between the BBMT and MMT appears to separate terranes that are not likely to be exotic in the sense of their Phanerozoic counterparts. Other Archean provinces do appear to contain crustal blocks with different isotopic signatures (e.g. West Greenland, India, South Africa). The use of the term exotic, therefore, must be cautious in situations where geographic indicators such as paleontologic and/or paleomagnetic data are not available. In these cases, isotopic signatures are one of the most useful features for assessing overall genetic relations amongst geologically distinct terranes.

Evidence for Proterozoic mylonitization in the northwestern Wyoming province

The 3-km-thick Madison mylonite zone encompasses the contact between the Cherry Creek Metamorphic Suite and the pre-Cherry Creek metamorphic complex in the southern Madison Range of southwest Montana. In the shear zone, greenschist- and epidote-amphibolite-facies assemblages overprint earlier amphibolite- and granulite-facies assemblages. Large-scale metasomatic conversion of granitic gneiss to mylonite gneiss increased siderophile (Fe, Mn, Mg, P, and Ca) elements and decreased K, Al, and Si in the mylonite gneiss, suggesting extensive fluid flux in the Madison mylonite zone. Southeast-dipping foliations adjacent to the Madison mylonite zone are rotated about their northeastern strike through the vertical to northwest-dipping orientations, indicating southeast-directed thrusting of the pre-Cherry Creek metamorphic complex over the Cherry Creek Metamorphic Suite. Complete stratigraphic continuity of Cherry Creek units within the mylonite zone shows the predominance of ductile shearing. Ductile thrusting is also indicated by asymmetric fabrics, stretching lineations, finite strain, and metamorphic gradients within the shear zone. If the foliations were passively rotated during simple shear, a minimum of 10 km of distributed ductile slip occurred in the Madison mylonite zone. 40Ar/39Ar thermochronology of hornblende and muscovite adjacent to the Madison mylonite zone records a Late Archean (2.5 Ga) cooling event. Within the shear zone, a discordant 40Ar/39Ar muscovite spectrum is consistent with initial cooling to argon closure at about 1.9 Ga with subsequent disturbance at about 1.6 Ga. Hornblendes from the Madison mylonite zone show highly discordant argon-loss spectra with temperature steps starting at 1.8 Ga and ending at 2.5 Ga. These argon age spectra indicate Early Proterozoic heating and argon loss in the Madison mylonite zone, resetting and subsequent retrogressing of muscovite but only partial resetting of hornblende due to its higher argon closure temperature. This study strongly suggests Early Proterozoic (1.80-1.90 Ga and possibly later) thrusting in the Madison mylonite zone, which may be correlative with retrograde metamorphism and Early Proterozoic isotopic resetting northwest of the Madison Range. The Madison mylonite zone is probably a foreland thrust zone on the margin of a major compressional orogen of Early Proterozoic age that reworked the Archean basement of the northwestern Wyoming province.

Comparison of the 1959 Hebgen Lake, Montana and the 1983 Borah Peak, Idaho, earthquakes from geodetic observations

Abstract We deduce the fault geometry, coseismic slip, and moment for two of the largest historic earthquakes that have occurred in the Basin and Range of the Western United States: the M = 7.3 1959 Hebgen Lake, Montana, earthquake and the M = 6.9 1983 Borah Peak, Idaho, event. Newly augmented data sets of vertical deformation from geodetic leveling and from lake shoreline changes were modeled by simple dislocations in an elastic half-space. The rms signal-to-noise ratio is 12 for the Hebgen Lake data and 38 for the Borah Peak set. The residuals for both models are about twice as large as the noise. The Hebgen Lake earthquake struck on the 15- to 25-km-long en-echelon Hebgen and Red Canyon faults, dipping 45° to 50° and extending to a depth of 10 to 15 km. The 7.0 and 7.8 m of dip-slip on these faults produced a combined moment of 1.2 × 10 20 N-m. The dip of the Red Canyon fault may decrease slightly with depth (in a listric manner), abutting the planar Hebgen fault at a depth of 8 km. In addition, up to 1 m of deep slip occurred on the Holocene segment of the adjacent Madison Range fault, 10 km west of the Hebgen fault. The Borah Peak segment of the Lost River fault was found to dip 49°. Slip of 2.1 m occurred at the south fault end, extending to a depth of 14 km; 1.4 m of slip occurred at the north end, where the fault reached only to 6 km depth. A listric fault shape is not permitted by the geodetic data at Borah Peak. Both the Hebgen-Red Canyon and the Lost River faults are high-angle and nearly planar, despite the much greater age and length of the Lost River fault in comparison to the en-echelon Hebgen faults. The chief difference between the earthquakes is the 3- to 4-fold higher slip at Hebgen relative to Borah Peak and all other well-studied Basin and Range shocks. Thrust faults located close to these active normal faults must either dip steeply at depth or were not reactivated.