David R. Lageson

Paleomagnetic and isotopic dating of thrust-belt deformation along the eastern edge of the Helena salient, northern Crazy Mountains Basin, Montana

Thrust-belt deformation along the eastern edge of the Disturbed Belt in the Helena salient of Montana has not been well dated owing to a lack of syn- or postorogenic strata. In situ paleomagnetic data from alkaline intrusions exposed in the easternmost folds of the salient in the northern Crazy Mountains Basin, previously described as pre-, syn-, or posttectonic with respect to deformation, are well grouped (Dec. = 343°, Inc. = 61°, α95 = 5.0°, k = 46, n = 16 sites); a negative fold test is significant at minimally 95% confidence. Intrusion and magnetization acquisition thus postdate fold and thrust deformation. K-Ar age determinations of selected intrusions range from 52 to 48 Ma. Paleomagnetic and isotopic age data, combined with stratigraphic information, indicate that folding in the northern Crazy Mountains Basin is middle to late Paleocene in age. This age is in agreement with suggested dates for deformation in the northern Montana Disturbed Belt but is recognizably older than the youngest episodes of frontal deformation in the Utah-Idaho-Wyoming salient. Data from this study and existing structural and stratigraphic information demonstrate that deformation in the overthrust belt and foreland provinces of southwest Montana overlapped temporally and spatially, ranged from Late Cretaceous to earliest Eocene in age, and progressed from west to east through time.

Influence of Late Cretaceous magmatism on the Sevier orogenic wedge, western Montana

Late Cretaceous large-volume pluton emplacement and accompanying volcanism within the evolving western Montana thrust wedge may have played important roles in determining the geometric and kinematic development of the thrust wedge, thereby influencing patterns of sediment dispersal and subsidence in the adjacent foreland basin. Intrusion of the Upper Cretaceous Boulder batholith and coeval eruption of the Elkhorn Mountains Volcanics were focused mainly at the trailing margin of the Lombard-Eldorado allochthon. The resultant thick (16–17 km) igneous culmination evolved over a relatively short interval (80–70 Ma), thickening the orogenic wedge to the point of supercritical taper and facilitating continued motion along the Lombard-Eldorado thrust system, thrust imbrication at the wedge toe, and forelandward translation of the Helena salient. Delivery of eroded volcaniclastic detritus from the thickened thrust wedge and accelerated basin subsidence due to thrust loading resulted in accumulation of a thick (4–5 km) sequence of Campanian-Maastrichtian volcaniclastic strata in the foreland basin (Livingston Group). The structural and sedimentological effects of this structural-magmatic culmination are similar to those of basement-cored culminations elsewhere in the Sevier orogen and to the Neogene central Andean orogenic wedge.

Microbialite response to an anthropogenic salinity gradient in Great Salt Lake, Utah

A railroad causeway across Great Salt Lake, Utah (GSL), has restricted water flow since its construction in 1959, resulting in a more saline North Arm (NA; 24%-31% salinity) and a less saline South Arm (SA; 11%-14% salinity). Here, we characterized microbial carbonates collected from the SA and the NA to evaluate the effect of increased salinity on community composition and abundance and to determine whether the communities present in the NA are still actively precipitating carbonate or if they are remnant features from prior to causeway construction. SSU rRNA gene abundances associated with the NA microbialite were three orders of magnitude lower than those associated with the SA microbialite, indicating that the latter community is more productive. SSU rRNA gene sequencing and functional gene microarray analyses indicated that SA and NA microbialite communities are distinct. In particular, abundant sequences affiliated with photoautotrophic taxa including cyanobacteria and diatoms that may drive carbonate precipitation and thus still actively form microbialites were identified in the SA microbialite; sequences affiliated with photoautotrophic taxa were in low abundance in the NA microbialite. SA and NA microbialites comprise smooth prismatic aragonite crystals. However, the SA microbialite also contained micritic aragonite, which can be formed as a result of biological activity. Collectively, these observations suggest that NA microbialites are likely to be remnant features from prior to causeway construction and indicate a strong decrease in the ability of NA microbialite communities to actively precipitate carbonate minerals. Moreover, the results suggest a role for cyanobacteria and diatoms in carbonate precipitation and microbialite formation in the SA of GSL.

Seismotectonics of the 20 August 1999 Red Rock Valley, Montana, Earthquake

On 20 August 1999, a magnitude 5.3 earthquake occurred in southwestern Montana, ending a 25-year hiatus for magnitude 5+ seismicity in Montana. This earthquake occurred in the central part of the Red Rock Valley, a northwest-trending graben bounded by late Pleistocene and Holocene faults. A focal depth of 12.4 km and a normal-faulting focal mechanism suggest that this earthquake resulted from continued graben development, although not along graben-bounding faults mapped at the surface. The 1999 Red Rock Valley earthquake occurred near the northern end and within the footwall block of the northeast-dipping Red Rock fault. We deployed a temporary network close to the mainshock epicenter and located 65 aftershocks over 3 days, including a magnitude 4.0 aftershock on 26 August that allowed determination of P -wave travel-time delays for regional seismograph stations. Using these station delays to improve relative hypocenter locations, we recomputed hypocenter locations for >1000 Red Rock Valley area earthquakes that occurred since 1989. Relocated hypocenters avoid the Holocene portion of the Red Rock fault but do surround it. The mainshock epicenter location is close to that of a magnitude 5.0 earthquake in 1965.

Polyphase deformation, dynamic metamorphism, and metasomatism of Mount Everest’s summit limestone, east central Himalaya, Nepal/Tibet

New samples collected from a transect across the summit limestone of Mount Everest (Qomolangma Formation) show that multiple distinct deformational events are discretely partitioned across this formation. Samples from the highest exposures of the Qomolangma Formation (Everest summit) preserve a well-developed mylonitic foliation and microstructures consistent with deformation temperatures of ≥250 °C. Thermochronologic and microstructural results indicate these fabrics were ingrained during initial contractile phases of Himalayan orogenesis, when crustal thickening was accommodated by folding and thrusting of the Tethyan Sedimentary Sequence. In contrast, samples from near the base of the Qomolangma Formation (South Summit) preserve extensional shear deformation, indicate metasomatism at temperatures of ∼500 °C, and contain a synkinematic secondary mineral assemblage of muscovite + chlorite + biotite + tourmaline + rutile. Shear fabrics preserved in South Summit samples are associated with activity on the Qomolangma detachment, while the crystallization of secondary phases was the result of reactions between the limestone protolith and a volatile, boron-rich fluid that infiltrated the base of the Qomolangma Formation, resulting in metasomatism. The 40 Ar/ 39 Ar dating of synkinematic muscovite indicates the secondary assemblage crystallized at ca. 28 Ma and that shear fabrics were ingrained at ≥18 Ma. This paper presents the first evidence that Everest’s summit limestone records multiple phases of deformation associated with discrete stages in Himalayan orogenesis, and that the structurally highest strand of the South Tibetan detachment on Everest was initially active as a distributed shear zone before it manifested as a discrete brittle detachment at the base of the Qomolangma Formation.

Remagnetization and folding in the frontal Montana Rocky Mountains

Local-scale folds within the Mississippian Madison Group of the frontal Montana Rockies preserve pre- and synfolding remagnetization data. Paleomagnetic results display inclinations of ∼70°, in contrast to the expected shallower directions for North American Mississippian rocks. The magnetization is chemical in origin, preserved in superparamagnetic to single-domain magnetite grains from fluid activity. Magnetic intensity results in the study area suggest that mineralization was more prevalent in the interior of the fold-and-thrust belt and diminished toward the east, resulting in lower intensities from less magnetite growth in the very frontal portions of the belt into the foreland. Fold test results of individual folds show syn- and prefolding remagnetizations as a function of location across the belt, with synfolding results in more westerly locations and prefolding results in the most frontal folds of the belt. By comparing our synfolding results with previously determined deformation ages for the Rocky Mountains, an Eocene (53.6 Ma) age for the remagnetization can be assigned. Based on the relative timing of remagnetization, a spatial pattern of folding in the study area is revealed. Major folding commenced (i.e., synfolding magnetization) during an Eocene remagnetization event, while the most frontal portion remained undeformed (i.e., prefolding magnetization) and was subsequently folded after regional remagnetization.

Subsurface structural and mineralogical characterization of the Laramide South Prairie fault in the Stillwater Complex, Beartooth Mountains, Montana

Subsurface analysis of the South Prairie fault, a Laramide basement backthrust located in the Stillwater Complex of the Beartooth Mountains, Montana, has allowed the heretofore unparalleled physical-chemical transect study of a deep subsurface Laramide fault. Fracturing exhibits a bimodal network interpreted as Riedel R and R′ fractures and, along with alteration, increases toward the heavily altered and cataclasized core zone. The width of the core zone is dependent on the host rock. Mineralogy varies from unaltered norite and gabbronorite in the host rock to characteristic alteration products of clinozoisite and serpentine within the damage zone, with minor tremolite. Veins are composed of Ca-stilbite and minor late-stage carbonate with talc. The core zone contains abundant serpentine/chlorite. Plagioclase is observed to withstand heavy stable fracturing and minor alteration to clinozoisite at grain boundaries before further alteration to clinozoisite. Orthopyroxene readily undergoes serpentinization at the same conditions. Evidence of pre-, syn-, and postkinematic fluids is abundant and consistent with the estimated South Prairie fault permeability. These observed characteristics suggest synkinematic conditions of temperature ≤300 °C and pressure <400 MPa, consistent with brittle to brittle-plastic deformation, which may have been ideal for chemical ore remobilization into the South Prairie fault via the fracture network and hydrothermal fluids.

Stewart Peak Culmination, Idaho-Wyoming Thrust Belt, as Compared with Other Fold-and-Thrust Belt Culminations: ABSTRACT

The Stewart Peak culmination, located in the northern Salt River Range of the Idaho-Wyoming thrust belt, is an anticlinorium in the hanging wall of the Absaroka thrust fault. The culmination is topographically and structurally higher than areas to the north or south and consists of the oldest rocks exposed in the thrust belt. Rocks from the lower part of the Absaroka thrust sheet, ranging in age from Middle Cambrian to Mississippian, are stacked by an anastomosing network of imbricate thrust faults. Fold geometries include kink, chevron, and open concentric forms which deformed by a flexural-slip mechanism. Structural culminations are an important and predictable component of most fold-and-thrust belts. Down-plunge projections from culminations into adjacent depressions are often the key to unraveling complex structural relations. Several factors may contribute to the development of a culmination. Surface geologic mapping, integrated with down-plunge projections and geophysical data, indicate that the Stewart Peak culmination is the result of polyphase uplift and arching of the Absaroka thrust sheet by motion on younger and structurally lower thrust faults, End_Page 736------------------------------ namely the Murphy and Firetrail thrusts. These younger faults are interpreted to sole into the overlying Absaroka thrust, forming a subsurface duplex zone which may have considerable oil and gas potential. In addition, magnetic data suggest that the Stewart Peak culmination may be positioned over the northwest continuation of the Moxa arch, an anticlinal flexure of authochthonous basement which formed contemporaneously with thin-skinned thrusting. The Stewart Peak culmination is compared to structural culminations in the Canadian Rockies and the southern Appalachian orogene of Virginia and Tennessee. Culminations are prospective areas for oil and gas, and traps may be formed in a variety of geometric configurations. End_of_Article - Last_Page 737------------

Structural Geology of Northern Salt River Range, Idaho-Wyoming Thrust Belt--Preliminary Report: ABSTRACT

The northern Salt River Range is the structural culmination of the Absaroka-St. Johns thrust complex. The Stewart Peak quadrangle, located on the culmination, has been mapped to gain an understanding of the nature of the thrusts and folds in this part of the Idaho-Wyoming thrust belt. Rocks ranging in age from Middle Cambrian through Late Cretaceous are interleaved in a complex array of imbricate thrust faults and asymmetric folds. Major thrust faults in the Stewart Peak quadrangle include the Absaroka, Murphy, and Firetrail. Imbricate thrusts in the hanging wall of the Absaroka include the Star, Stewart, and four imbricate slices at the north-central margin of the quadrangle which may correlate with the St. Johns complex in the Snake River Range. The Grand Valley fault bounds the range near the west margin of the quadrangle where fanglomerates of probable Tertiary age (Pliocene?) are offset against Middle and Upper Cambrian strata. Several conclusions may be proposed regarding the northern Salt River Range. (1) Cataclasis occurs on a scale much greater than previously reported because deeper and more intensely deformed levels of the thrust belt are exposed relative to thrusts cropping out east and south in the Idaho-Wyoming salient. (2) Deformational intensity increases downward through the Paleozoic succession as the basal Absaroka decollement in the Cambrian Wolsey Shale is approached. (3) Stratigraphic thicknesses for units below the Mississippian Madison Group are tectonically thickened by ubiquitous small-scale thrust slivers (each with a few centimeters or more offset), stylolites, and small-scale folds. Stratigraphic correlations and isopach studies based on the present distribution of tectonically thicke ed Paleozoic units should not be made in this part of the thrust belt. (4) The Stewart Peak area represents a structural culmination in which the roots of the Absaroka thrust have been exposed, possibly from thrusting over a basement arch. In this regard, isopach trends of Cretaceous rocks east of the Darby-Hogsback thrust suggest that the Moxa arch may continue northwestward beneath the thrust belt in alignment with the Stewart Peak culmination. In addition, several structural discontinuities within the thrust belt northwest of LaBarge suggest the influence of a basement upwarp. The Stewart Peak culmination may therefore reflect a deeper structural level of exposure owing to thrusting over a basement arch above the regional level of decollement. This interpretation has important ramific tions regarding the structural control of potential oil and gas reservoirs beneath the Absaroka thrust. End_of_Article - Last_Page 833------------