Post-alleghanian unroo fing history of the appalachian basin, Pennsylvania, based on apatite fission-track analysis

ABSTRACTS Beyond William Smith: an Apatite Fission Track Map of Great Britain. Anthony J. Hurford*, Cherry L.E. Lewis*, Andrew J. Yelland*, Andrew Carter* and Paul F. Green.** *London Fission Track Research Group, University College London, Gower Street, London WC1E 6BT, UK. **Geotrack International, University of Melbourne, Parkville, Victoria 3052, Australia. In his travels during the 18th and early 19th Centuries as a surveyor for the construction of canals and bridges, William Smith was the first to recognise the importance of fossils in establishing a chronology of rock strata, and to publish the first geological map of the British Isles. Following Smith's classic work, many of the fundamental tenets of geology were established by pioneer geologists working in Great Britain, with detailed, large-scale geological mapping of the country being completed at an early date. Despite these critical early studies and 200 years of subsequent detailed research into seemingly every aspect of British geology, current apatite fission track analysis is revealing new insights into the thermo-tectonic development of Britain during the late Mesozoic and Cenozoic. The position of Britain between the extensional regime of the rifting northeast Atlantic and the Alpine compression of the African-Eurasian plate collision has been noted previously to have resulted in a series of discrete axes of sedimentary basin inversion. However, more than 300 apatite fission track analyses on crystalline and sedimentary samples from throughout England, Scotland, Wales and Ireland has identified a much more regional phenomenon at this time, with Cretaceous burial, followed by extensive Tertiary erosion occurring over most of Britain. The recognition of this event has major implications for the maturation of hydrocarbons in both onshore and •offshore exploration areas.

POST-ALLEGHANIAN UNROOFING HISTORY OF THE APPALACHIAN BASIN, PENNSYLVANIA, BASED ON APATITE FISSION-TRACK ANALYSIS BLACKMER, Gale C., Department of Geosciences, The Pennsylvania State University, University Park, PA, 16802, and OMAR, G.I., Geology Department, University of Pennsylvania, Philadelphia, PA, 19104. The post-Alleghanian unroofing history of the Appalachian Basin in Pennsylvania was investigated by combining the results of fission-track thermal history models and heat flow models. Maximum temperatures and burial depths were constrained by vertical vitrinite reflectance profiles and fluid inclusion trapping temperatures and pressures. Models were cast within the context of regional geology and tectonics to insure that they were reasonable. Cooling histories were modeled from the results of apatite fission-track analysis on 29 samples from Paleozoic sedimentary rocks. Fission-track ages of 111-184 Ma with mean track lengths of 10.7-13.1 ~rn and unimodal, negatively skewed length distributions are best modeled by slow cooling beginning after the Alleghanian Orogeny, at about 240-250 Ma, with burial and unroofing as the primary influence on thermal history. To facilitate investigation of the unroofing behavior of the basin, burial histories were generated from fissiontrack cooling histories using a one-dimensional heat flow model. Regions within the basin having related cooling histories were identified through the age-mean length relationship. These data separate into two trends, with older ages found in structurally high areas and younger ages in structural depressions. This regional variation in fission-track age is attrib~tted to the interaction of burial depth, maximum temperature, and initial unrGofing rate. Unroofing histories can be divided into three periods related to flexural behavior of the foreland: 1. L. Permian-Jurassic: rapid unroofing in response to erosional removal of orogenic load; 2. Jurassic-Miocene: little to no unroofing due to subsidence of the remaining load below sea level on the new continental margin, preventing further removal of load and suppressing flexural rebound; 3. Miocene-present: rapid unroofing due to an event of unknown nature.

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