Mary K. Roden

Thermal evolution of the Gangdese batholith, southern Tibet: A history of episodic unroofing

The Gangdese batholith, southern Tibet, was part of an Andean-type arc at the southern margin of Asia prior to the collision of India and Asia at approximately 50 to 40 Ma. Fission-track and 40Ar/39Ar analyses of 28 rocks from 10 Gangdese granitoid plutons along an ∼250 km length of the batholith in the Lhasa region provide a detailed understanding of the age and the postcrystallization erosional and tectonic history of these rocks. These data suggest a range of ages for these plutons of 94 to 42 Ma, with the majority being of Tertiary age. The postcrystallization cooling histories of all of these plutons are characterized by marked discontinuities. We conclude that most of these discontinuities, and all of them after 40 Ma, reflect tectonic changes that produced brief pulses of rapid erosion which were distributed in both space and time. In addition to the initial cooling of hot magma against cold country rock, all of the rocks we studied showed evidence for at least one subsequent episode of rapid cooling, dropping many tens of degrees in a few million years. Conversely, these plutons all experienced intervals during which they cooled very slowly or not at all; these slow-cooling intervals lasted from 5 to 50 million years. Our data indicate that since the collision between India and Asia began, response to continued convergence has been quite variable in even this relatively small area. The data reported here are consistent with a recently proposed model of Oligo-Miocene crustal shortening along the Gangdese Thrust system in this area.

Etchable length reduction of induced fission tracks in apatite at room temperature (≈ 23°C): Crystallographic orientation effects and “initial” mean lengths

Abstract Etchable length reduction of thermal-neutron induced fission tracks is observed in four apatites that experienced temperatures no greater than approximately 23°C subsequent to track formation. In each apatite, the mean etchable fission track lengths parallel ( l c ) and perpendicular ( l a ) to the crystallographic c -axis as well as the arithmetic (conventional) mean ( l m ) decrease by approximately 0.5 μm during the time interval ≈ 10 min-≈ 3 weeks after irradiation. Two crystallographic orientation dependencies of mean etchable fission track length at room temperature are identified: (a) l c > l a in all cases (tracks etched for 25 s at 23°C in 5 M HNO 3 ) and (b) l a decreases approximately twice as fast as l c . These measurements strongly support previous suggestions that natural charged-particle tracks in crystalline materials, including fission tracks, undergo slow annealing at low ambient temperatures. The observed sensitivity of induced fission tracks in apatite to low-temperature annealing requires that care be taken to avoid heating apatites before and during track etching for so-called “initial” mean track length measurements.

Apatite fission-track thermochronology of the Pennsylvania Appalachian Basin

Thirty-four apatite fission-track apparent ages and twenty-four track length distributions for ash bed samples from the Valley and Ridge Province and Upper Devonian to Upper Pennsylvania sedimentary samples from the Allegheny Front and Allegheny Plateau of Pennsylvania suggest that these regions represent different thermal (uplift) regimes as well as different structural provinces. The Valley and Ridge Province Tioga and Kalkberg ash bed samples yield apatite fission-track apparent ages and track length distributions that indicate early post-Alleghanian (285-270 Ma) cooling and unroofing that began at ∼250 Ma. Assuming a geothermal gradient of 25°C km−1, a burial depth of at least 3.4 km can be estimated for all the Pennsylvania samples. At the Allegheny structural front and on the western Allegheny Plateau, the apatite fission-track apparent ages (<150 Ma) and track length measurements indicate a Late Jurassic-Early Cretaceous thermal event for these samples possibly resulting from a higher geothermal gradient coinciding with kimberlite intrusion at this time along the Greene-Potter Fault Zone. In northeast Pennsylvania on the Allegheny Plateau, the Upper Paleozoic sedimentary samples yield apatite fission-track apparent ages ≤180 Ma. Narrow track length distributions with long mean lengths (13–14 μm) and small standard deviations (1.3 μm) suggest rapid cooling from temperatures >110°C during the Middle Jurassic-Early Cretaceous for this part of Pennsylvania. This is consistent with the suggested uplift history of the Catskill Mountain region in adjacent New York State.

The Absolute Age of the Eifelian Tioga Ash Bed, Pennsylvania

The Tioga ash bed, bed B of the Zeigler pit, Union County, Pennsylvania, lies in the Polygnathus costatus costatus Zone of the Eifelian Stage of the Middle Devonian Series. The 27.5-cm-thick bed contains igneous zircon and monazite crystals that have been dated by precise U-Pb techniques. Zircons are characterized by inherited Pb that occurs as barely perceptible interior regions enriched in inclusions. The zircons are discordant and do not reveal the igneous age of the bed. In contrast, high quality monazite crystals lack any evidence of inheritance or loss of Pb, and multiple analyses have overlapping

Evolution of the Continental Margin of Western India: New Evidence from Apatite Fission-Track Dating

^{207}Pb/^{235}U

A Comparison of Fission-Track Ages of Apatite and Zircon to the K/Ar Ages of Illite-Smectite (I/S) from Ordovician K-Bentonites, Southern Appalachian Basin

ages of

Dependence of fission track annealing kinetics on apatite crystal chemistry

390.0 \pm 0.5 Ma