Karl S. Kellogg

Emplacement temperatures of unsorted and unstratified deposits of volcanic rock debris as determined by paleomagnetic techniques

Unsorted and unstratified deposits of volcanic rock debris typically flank recently active stratovolcanoes. It is often difficult, using standard geologic procedures, to establish whether a particular deposit was emplaced by a pyroclastic flow, lahar, rock avalanche, or glacier. Determination of the emplacement temperatures of clasts contained in the deposit aids in discriminating among some of these possibilities. The emplacement temperature of a clast can be estimated by analyzing its thermoremanent magnetization. To do this, oriented samples of the clasts are submitted to progressive thermal demagnetization; the directions and magnitudes of the resulting residual remanent vectors provide the information necessary to estimate the temperatures at which the clasts were emplaced. Studies of samples that were given artificial emplacement temperatures reveal that estimates are within ±25 °C of the actual values. The temperature range within which estimates are possible depends on the thermoremanent magnetization acquisition curve of a clast. Data obtained from deposits of volcanic rock debris from Mount St. Helens, Washington, indicate that for andesitic and dacitic rocks, the range is roughly 100 to 550 °C. The procedure probably can be applied to other geologic problems that require temperature information.

Tectonic controls on a large landslide complex: Williams Fork Mountains near Dillon, Colorado

Abstract An extensive (∼ 25 km2) landslide complex covers a large area on the west side of the Williams Fork Mountains in central Colorado. The complex is deeply weathered and incised, and in most places geomorphic evidence of sliding (breakaways, hummocky topography, transverse ridges, and lobate distal zones) are no longer visible, indicating that the main mass of the slide has long been inactive. However, localized Holocene reactivation of the landslide deposits is common above the timberline (at about 3300 m) and locally at lower elevations. Clasts within the complex, as long as several tens of meters, are entirely of crystalline basement (Proterozoic gneiss and granitic rocks) from the hanging wall of the Laramide (Late Cretaceous to Early Tertiary), west-directed Williams Range thrust, which forms the western structural boundary of the Colorado Front Range. Late Cretaceous shale and sandstone compose most footwall rocks. The crystalline hanging-wall rocks are pervasively fractured or shattered, and alteration to clay minerals is locally well developed. Sackung structures (trenches or small-scale grabens and upslope-facing scarps) are common near the rounded crest of the range, suggesting gravitational spreading of the fractured rocks and oversteepening of the mountain flanks. Late Tertiary and Quaternary incision of the Blue River Valley, just west of the Williams Fork Mountains, contributed to the oversteepening. Major landslide movement is suspected during periods of deglaciation when abundant meltwater increased pore-water pressure in bedrock fractures. A fault-flexure model for the development of the widespread fracturing and weakening of the Proterozoic basement proposes that the surface of the Williams Range thrust contains a concave-downward flexure, the axis of which coincides approximately with the contact in the footwall between Proterozoic basement and mostly Cretaceous rocks. Movement of brittle, hanging-wall rocks through the flexure during Laramide deformation pervasively fractured the hanging-wall rocks.

Paleomagnetic evidence for oroclinal bending of the southern Antarctic Peninsula

Paleomagnetic results from the investigation of 13 magnetically stable units (92 oriented rock samples) of Upper Cretaceous (“Andean”) plutons and dikes from the Orville Coast and eastern Ellsworth Land, Antarctica, define a mean direction of magnetization of I = −77°, D = 51° (α 95 = 5.9°), with a paleomagnetic pole at 71°S, 165°W. The sampled units were emplaced after the Late Jurassic to Early Cretaceous intense folding associated with subduction along the western side of the Antarctic Peninsula. In addition, all sampled intrusive rocks are normally magnetized and are believed to have been emplaced during the Late Cretaceous period of predominantly normal polarity. There is no evidence of post-emplacement remagnetization. Unlike rocks from other Andean paleomagnetic collecting localities on the Antarctic Peninsula, whose mean declinations are oriented approximately north, the mean declination of samples from the Orville Coast and eastern Ellsworth Land is rotated 51° clockwise from north. Uncertainty in declination at the 95% confidence level (δ 95 ) is ±27°. The data support the conclusion that the southern bend of the S-shaped Antarctic Peninsula was formed after Late Cretaceous time. Early Tertiary right-lateral transform faulting across the base of the Antarctic Peninsula may have produced this major oroclinal bend. Data from four localities (21 samples; I = −79°, D = 30°, α 95 = 4.3°) in Upper Jurassic(?) massive rhyodacite porphyry lava flows in the northern part of the area are similar to those of the Andean igneous rocks. Although the evidence is not conclusive, it seems most probable that the porphyry was magnetically reset by Late Cretaceous plutonism.

Neogene basins of the northern Rio Grande rift: partitioning and asymmetry inherited from Laramide and older uplifts

Abstract Three asymmetric Neogene basins in the northern Rio Grande rift of New Mexico and Colorado — the San Luis basin, the upper Arkansas River graben, and the Blue River graben — are tilted toward large flanking normal faults and lie astride the similarly asymmetric Late Cretaceous–early Tertiary (Laramide) San Juan–San Luis, Sawatch, and Front Range–Gore Range uplifts, respectively. The steep, thrust-faulted side of each uplift is on the same side as the down-rotated side of each of the Neogene basins. In addition, the direction of stratal tilt changes northward across the Villa Grove accommodation zone from east in the San Luis basin to west in the upper Arkansas River graben. This accommodation zone coincides approximately with the northward change from the east-directed San Juan–San Luis uplift to the west-directed Sawatch uplift. These observations, supported by seismic-reflection studies across the San Luis basin and studies of several other superimposed pairs of rift basins and Laramide uplifts, suggest that the basin-bounding normal faults are listric and merge at depth with the older thrusts, which are also listric and root into the crust at about 15–16 km. The Blue River graben is complicated by lack of basin fill and a thrust history along the west side of the Gore Range that is at least as old as late Paleozoic. Nonetheless, the Neogene valley is demonstrably tilted west and lies astride an overall west-directed thrust system, similar to other thrust-and-basin relationships in the northern Rio Grande rift.

The Mesoproterozoic Beaverhead Impact Structure and Its Tectonic Setting, Montana‐Idaho: 40Ar/39Ar and U‐Pb Isotopic Constraints

New 40Ar/39Ar and uranium‐lead (U‐Pb) zircon data from the Beaverhead impact structure, first identified by extensive shatter coning of Proterozoic quartzite and gneiss from the Beaverhead Mountains near the Montana‐Idaho border, indicate that the structure formed at or after 900 Ma. The 40Ar/39Ar age spectra from fine‐grained muscovite and biotite from a breccia zone in high‐grade gneiss show significant argon loss but yield dates for highest‐temperature steps that cluster between 899 and 908 Ma. The dated minerals probably formed by recrystallization of impact glass, so on both geologic and isotopic grounds, the dates probably represent the minimum age of impact. U‐Pb data for zircons from the same breccia are strongly discordant and yield an upper intercept apparent age of \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

Laramide to Holocene structural development of the northern Colorado Front Range

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A landslide in Tertiary marine shale with superheated fumaroles, Coast Ranges, California

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