Lawrence W. Snee

Dating of major normal fault systems using thermochronology: An example from the Raft River detachment, Basin and Range, western United States

Application of thermochronological techniques to major normal fault systems can resolve the timing of initiation and duration of extension, rates of motion on detachment faults, timing of ductile mylonite formation and passage of rocks through the crystal-plastic to brittle transition, and multiple events of extensional unroofing. Here we determine the above for the top-to-the-east Raft River detachment fault and shear zone by study of spatial gradients in 40Ar/39Ar and fission track cooling ages of footwall rocks and cooling histories and by comparison of cooling histories with deformation temperatures. Mica 40Ar/39Ar cooling ages indicate that extension-related cooling began at ∼25–20 Ma, and apatite fission track ages show that motion on the Raft River detachment proceeded until ∼7.4 Ma. Collective cooling curves show acceleration of cooling rates during extension, from 5–10°C/m.y. to rates in excess of 70–100°C/m.y. The apparent slip rate along the Raft River detachment, recorded in spatial gradients of apatite fission track ages, is 7 mm/yr between 13.5 and 7.4 Ma and is interpreted to record the rate of migration of a rolling hinge. Microstructural study of footwall mylonite indicates that deformation conditions were no higher than middle greenschist facies and that deformation occurred during cooling to cataclastic conditions. These data show that the shear zone and detachment fault represent a continuum produced by progressive exhumation and shearing during Miocene extension and preclude the possibility of a Mesozoic age for the ductile shear zone. Moderately rapid cooling in middle Eocene time likely records exhumation resulting from an older, oppositely rooted, extensional shear zone along the west side of the Grouse Creek, Raft River, and Albion Mountains.

The 40Ar/39Ar ages and tectonic setting of the Middle Eocene northeast Nevada volcanic field

Widespread middle to late Eocene calc-alkalic volcanism, which formed the Northeast Nevada Volcanic Field, marks the earliest Tertiary volcanism in the northern Basin and Range. The central part of this major field in northeast Nevada and adjacent Utah is herein defined by 23 40Ar/39Ar ages that range from 42.6 to 39.0 Ma, rock chemistry from 12 localities, stratigraphic position of the volcanic rocks above a regional middle Eocene unconformity, volcanic setting, and lithology. The type area is at Nanny Creek, in the northern Pequop Mountains, Nevada, where rhyolite ash flow tuffs are overlain by a thick section of intercalated andesitic to dacitic flows and flow breccias and rhyolite ash flow tuffs. The intermediate composition rocks are locally derived throughout the volcanic field, whereas the sources for rhyolite ash flow tuffs are unknown. The uniform and widespread occurrence of the andesitic and dacitic flows and flow breccias strongly suggests that the upper crust was perforated by intermediate composition magma across the entire region. In the central part of the field the middle Eocene volcanic rocks rest with depositional angular discordance on deformed middle Paleozoic to Triassic strata; ostracode-bearing limestone, probably of early Eocene age, is locally present below the volcanic rocks. In the western and southeastern parts of the field these middle Eocene volcanic rocks rest with depositional angular discordance on lower Eocene lacustrine strata of the Elko and White Sage Formations, respectively. This angular discordance documents a middle Eocene deformational event previously unrecognized in the region.

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.

Paleomagnetism of the Middle Proterozoic Laramie anorthosite complex and Sherman Granite, southern Laramie Range, Wyoming and Colorado

We present the results of a combined paleomagnetic and 40Ar/39Ar geochronologic investigation of the Middle Proterozoic Laramie anorthosite complex and Sherman Granite in the southern Laramie Range of Wyoming and Colorado. Anorthosites and monzosyenites of the Laramie anorthosite complex yield a well-defined characteristic magnetization of northeast declination (D) and moderate negative inclination (I), although antipodal normal and reverse polarity magnetizations are present at three sites. A grand mean direction from 29 of 35 sites in the complex is D = 44.6°, I = −48.7° (k = 77.4, α95 = 3.1°). Alternating field (AF) and thermal demagnetization behavior and rock magnetic experiments indicate that magnetization is carried by low-Ti titanomagnetite of single or pseudo-single domain character that occurs as elongate to rod-shaped inclusions in plagioclase and potassium feldspar. The Sherman Granite contains a dual polarity magnetization that is less well defined than that of the Laramie anorthosite complex but similar in declination and inclination (D = 53.1°, I= −48.1°, k = 46.5, α95 = 7.6°, n = 8/14 sites); rock magnetic data indicate the primary carrier of remanence in Sherman Granite is magnetite. The 40Ar/39Ar geochronologic data from Sherman Granite hornblende, biotite, and microcline indicate that subsolidus cooling was moderate to relatively rapid through the range of temperatures over which magnetization was blocked and that the age of remanence is about 1415 Ma. Microcline data indicate that the Laramie anorthosite complex and Sherman Granite have probably not been thermally remagnetized. Paleomagnetic poles from the Laramie anorthosite complex and Sherman Granite are indistinguishable at the 95% confidence level, and individual virtual geomagnetic poles (VGPs) from both units are combined to provide a mean pole at 215.0°E, 6.7°S (K = 46.9, A95 = 3.5°, N = 37 VGPs). The location of this pole is similar to paleomagnetic poles derived from 1480 Ma to 1450 Ma intrusions elsewhere in North America, but it plots significantly north of those from Middle Proterozoic sedimentary strata of the Belt Supergroup and Sibley Group. In addition to the characteristic magnetization, samples from some sites in Sherman Granite contain a remanence of southeast declination and shallow negative inclination (D = 154.9°, I = −16.0°, k = 90.6, α95 = 9.7°, n = 4 sites). This secondary magnetization was probably acquired during late Paleozoic time.

Stratigraphic, sedimentologic, and petrologic record of late Miocene subsidence of the central Oregon High Cascades

Pliocene to Holocene volcanics of the central Oregon High Cascades, largely confined to an intra-arc graben, bury older volcanic centers. Early High Cascade volcanism is recorded by late Miocene volcanics and sediments of the Deschutes formation that accumulated in a fluvial basin east of the Cascades. Aggradation in the adjacent fluvial basin occurred episodically when pyroclastic debris choked streams. Deschutes basin aggradation commenced near 7.4 Ma and was synchronous with extension-influenced Cascade magmatism dominated by basalts and basaltic andesites. Initial subsidence of the Cascades occurred at about 5.4 Ma in response to plate-margin processes. Structural isolation of the basin from ignimbrites and volcanism-induced sedimentation resulted in widespread development of superimposed paleosols in the basin. Subsequent mafic lavas erupted near the eastern margin of the initial depression were truncated by east-stepping, down-to-the-west faults at about 5.3 Ma.

Relation of peralkaline magmatism to heterogeneous extension during the middle Miocene, southeastern Nevada

Volcanism migrated southward in the northern Basin and Range province in the Oligocene and early Miocene to produce voluminous calcalkaline silicic ash flow tuffs. Alkaline volcanism became dominant by middle Miocene (17–14 Ma) as smaller volumes of rhyolite-trachyte-basalt suites were erupted from the relatively small Kane Springs Wash caldera complex including the Narrow Canyon, Boulder Canyon, and Kane Springs Wash calderas in southeastern Nevada. Only minor extension affected the Kane Wash area before the end of calcalkaline activity, but extension expressed by rate of progressive stratal tilt peaked (15–13.5 Ma) with peralkaline magmatism (14.7–14.4 Ma). Variations in distribution, degree, style, and timing of deformation demonstrate heterogeneous extension in the Kane Wash area. Only minor extension and tilting persisted post-middle Miocene (<12 Ma). All major eruptive sources overlap domains of rapid extension. Most of the eruptive volumes from the two oldest calderas of the complex apparently pooled within their calderas, creating outflow deficits. Denudation faulting associated with magmatic tumescence may have followed preexisting active extensional fault systems to unload magma chambers, thus triggering eruptions into structural depressions. Evolution of alkaline magmas is demonstrated by progressive increases in peralkalinity and high field strength elements such as Zr, Y, and Nb. Nd, Pb, and Sr isotopic compositions provide evidence that significantly less crustal interaction affected middle Miocene peralkaline magmas than pre-middle Miocene calcalkaline magmas. eNd values are −5 to −7 for peralkaline magmas and −7 to −11 for calcalkaline magmas; 208Pb/204Pb ratios are 38.2–38.6 for peralkaline magmas and 38.5–38.9 for calcalkaline magmas. Regional cooling, short duration of magmatism, small volumes of magma, and local extension caused less crustal interaction in peralkaline Kane Wash magmas than in earlier magmas. North of the Kane Wash area, older more voluminous calcalkaline magmas intruded hotter crust for a longer period and thus interacted with the crust to a greater degree in spite of synvolcanic extension.

40Ar/39Ar Geochronology and Geochemical Reconnaissance of the Eocene Lowland Creek Volcanic Field, West‐Central Montana

We report geochronological and geochemical data for the calc‐alkalic Lowland Creek volcanic field (LCVF) in west‐central Montana. 40Ar/39Ar age determinations show that the LCVF was active from 52.9 to 48.6 Ma, with tuff‐forming eruptions at \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Paleomagnetism of the Miocene intrusive suite of Kidd Creek : Timing of deformation in the Cascade arc, southern Washington

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