Charles W. Naeser

Recent rapid uplift in the Bolivian Andes: Evidence from fission-track dating

Apatite and zircon fission-track cooling ages constrain the Tertiary cooling and uplift history of the eastern Cordillera and Altiplano of Bolivia. Fission-track data are from two Triassic plutons and surrounding Paleozoic metasedimentary rocks in the eastern Andes north of La Paz. Zircon cooling ages indicate that the roof of the Huayna Potosi pluton was emplaced in the zircon partial annealing zone and that the Zongo pluton was emplaced entirely in the zircon total annealing zone. Apatite cooling ages for both plutons record uplift in the past 5-15 m.y., and zircon cooling ages from the Zongo pluton reflect uplift in the past 25-45 m.y. Uplift rates calculated by these apatite and zircon cooling ages suggest that uplift rates were 0.1-0.2 mm/yr between 20 and 40 Ma and increased significantly between 10 and 15 Ma. By 3 Ma, uplift rates may have been as high as 0.7 mm/yr.

Magnetic Polarity Stratigraphy And Mammalian Fauna Of The Deseadan (Late Oligocene-Early Miocene) Salla Beds Of Northern Bolivia

The Salla Beds contain a rich assemblage of Deseadan mammals that traditionally has been considered of early Oligocene age. These deposits, located 90-100 km SE of La Paz, Bolivia, consist of more than 540 m of principally fluviatile clays and silts with numerous interbedded tuffs. Paieomagnetic samples were collected from 104 sites spaced at stratigraphic intervals of about 6 m. As a result of thermal demagnetization (at 400°C or greater), unambiguous polarities were determined for 76 of the 104 originally sampled sites. The pattern of reversals, stage of evolution of the fossils, and preliminary isotopic dates (including a K/Ar age of

Unroofing history of a suture zone in the Himalaya of Pakistan by means of fission-track annealing ages

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Oligocene and Miocene metamorphism, folding, and low-angle faulting in northwestern Utah

Ma and zircon fission track ages of ca. 22 Ma) from interbedded tuffs suggest that the Salla Beds correlate to chrons C10 through C6 of the magnetic polarity time scale, or an absolute time interval from about 28.5 to 24 Ma. Pyrotherium and caviomorph rodents occur throughout the section; the primate Branisella is known to occur at about 27 Ma. The Salla faunas seem to represent an earlier part of the Deseadan than faunas of this Land Mammal Age from Argentina. These data suggest that the early Deseadan should be shifted significantly upwards (by ca. 10 m.y.) to late Oligocene-early Miocene time.

Middle Miocene tectonic development of the Transition Zone, Salta Province, northwest Argentina: Magnetic stratigraphy from the Metán Subgroup, Sierra de González

Abstract Volcanic sediments, now mostly bentonites and bentonitic mudstones, occur throughout the Late Neogene and Quaternary Siwalik Group of northern Pakistan. A number of these deposits have been dated by the fission-track method, utilizing zircon phenocrysts from these deposits, and provide the chronometric constraints upon which a paleomagnetic stratigraphy is developed for the Siwalik Group. Notable in the occurrence of these altered tuff horizons is an apparent mode in their stratigraphic development from approximately 3.0 to 1.5 m.y. B.P. which coincides with the period of activity of the Dacht-e-Nawar volcanic complex of east-central Afghanistan. Fission-track ages of certain tuffs for critical areas of northern Pakistan are reported herein.

Late Cenozoic paleomagnetism and chronology of Andean basins of Bolivia: Evidence for possible oroclinal bending

The uplift history of the Swat Valley and Hazara region of northwestern Pakistan has been established using 22 fission-track dates on apatite, zircon and sphene. A major fault, the Main Mantle Thrust (MMT) strikes east-west across the Swat Valley, separates regions of markedly differing fission-track age regimesm, and may be a suture zone separating an extinct island arc terrane on the north from the Indian plate to the south. Fission-track ages ranging from about 55 to 58 m.y. for sphene, 18 to 53 m.y. for zircon, and 9 to 17 m.y. for apatite were obtained from the region north of the MMT. To the south the fission-track age ranges are 20 to 25 m.y. for sphene, 17 to 26 m.y. for zircon, and 16 to 23 m.y. for apatite. Disparate zircon and sphene ages on each side of the MMT imply different cooling histories for each side of the fault prior to 15 m.y. Similar apatite ages on both sides of the fault imply similar cooling histories during the past 15 m.y. This may indicate that faulting ceased by 15 m.y. Mean uplift rates have been derived from the fission-track data using mainly the mineral-pair method. Uplift rates in the region north of the MMT increased from 0.07 to 0.20 mm/yr during the period 55 to 15 m.y. South of the fault, uplift rates averaged in excess of 0.70 mm/yr for the period 25 to 15 m.y. During the past 15 m.y. uplift across the MMT in the Swat Valley showsno discontinuities, ranging from 0.16 mm/yr in the south to 0.39 mm/yr in the north. A plausible interpretation for the fission-track uplift data has the MMT verging to the south with overthrusting taking place at a depth between 3.5 and 6.0 km, juxtaposing two terranes that were originally separated by a substantial, but unknown distance. In this model, regional uplift followed cessation of faulting just prior to 15 m.y.

Ages and stratigraphy of lower and middle Tertiary sedimentary and volcanic rocks of the central Cascades, Washington: Application to the tectonic history of the Straight Creek fault

Abstract Apatite and zircon separates from the Fish Canyon Tuff (K-Ar age, 27.9±0.7 Myr), San Juan Mtns., Colorado, have been given to over 50 laboratories for fission-track dating. Nineteen laboratories have reported fission-track ages that they have determined for apatites. Nine laboratories have reported their analysis of the zircons. The principal difference between the results reported by the laboratories reflects their choice of the decay constant. The laboratories which use a value of λ f ∼ 7.0 × 10 −17 yr −1 for the spontaneous-fission decay constant of 238 U, report an average age for the apatite of 28.5±0.7 Myr, and those using λ f ∼ = 8.4 × 10 −17 yr −1 report an average age of 23.6±1.0 Myr. The average fission-track age for the zircons is 28.4±0.7 Myr. Only laboratories which use λ f ∼ 7.0 × 10 −17 yr −1 reported zircon data.

The Application of Fission-Track Dating to the Depositional and Thermal History of Rocks in Sedimentary Basins

An area of 3,000 km 2 in and around the Grouse Creek Mountains and the Raft River Mountains exposes Precambrian, Paleozoic, and Triassic sedimentary rocks that were folded several times and displaced tens of kilometres on low-angle faults. Overturned folds and local imbrications indicate transport westward and northward during two episodes of metamorphic deformation and transport eastward after metamorphism. Metamorphic grade increases downward in the allochthonous sheets and autochthon and increases westward in the autochthon. Mineral grains are flattened into the horizontal plane, and shear strains increase upward, suggesting that the deformations were caused by gravity acting on a broadly heated dome. Rb-Sr dating of granitic plutons affected by the deformations indicates that (1) the area is underlain by adamellite, about 2.5 b.y. old, in which deformation decreased progressively downward; (2) the first metamorphic deformation probably ended before 38.2 ± 2.0 m.y. ago; and (3) the second metamorphic deformation was still underway 24.9 ± 0.6 m.y. ago. High-grade allochthonous rocks that lie on low-grade parts of the autochthon indicate as much as 30 km of eastward transport after metamorphism. Parts of the dome sagged to form broad basins 12 m.y. ago, and the coarse sediments and tuffs that accumulated in them were overrun by allochthonous sheets measuring at least 11 by 19 km. Two Rb-Sr mineral isochrons and several fission-track ages indicate that some parts of the area cooled below 400 °C only 10 m.y. ago.