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Laramide exhumation and heating in southeastern Arizona low-temperature thermal history and implications for zircon fission-track systematics /

by Riley, Brook Colleen

Abstract (Summary)
Disagreement regarding patterns of Laramide deformation and basin development in SE Arizona underlies the lack of a clear picture of exhumation and sediment recycling for syntectonic basins in this area. Fission-track (FT) ages of zircon from Mesozoic sandstones were analyzed to address the provenance and post-depositional thermal history of the synorogenic strata. Samples from strike-normal transects across the Laramide basin boundary within the Upper Jurassic-Upper Cretaceous section show a complex provenance, with significant recycling from the underlying rocks. FT peak ages from 14 Jura-Cretaceous sandstones include populations of 570-165, 140-82, and 68-42 Ma. Numerous older singlegrain ages (1000-600 Ma) are also present, indicating long-term sub-annealing temperatures (less than perhaps 180°C, up to 260°C, depending on single-grain response to heating) for portions of the source area. While most samples show a range of provenance ages, a small fraction of grains shows evidence for significant post-depositional thermal annealing. Burial depths were highly variable, but the cumulative thicknesses of units overlying the Jura- Cretaceous section was likely less than 2 km. Accordingly, the zircon FT age structure present in sandstone samples included in this study is not just the result of burial. Regionally, samples with Paleocene-Eocene reset peak ages coincide with proximity to magmatic bodies ranging from 75-40 Ma in age. Besides conductive heating, a hydrothermal system may have locally modified these rocks, and caused preferential resetting of certain zircons. Older grain ages present in all sampled units indicate little burial of the older Jurassic and Cretaceous rocks prior to recycling into the Fort Crittenden Formation (Upper Cretaceous); record little variation in the thermal signature of the recycled crustal detritus; and show that the older Jurassic and 1 Cretaceous rocks likely provided an important sediment source for the Fort Crittenden Formation. Prior work on the provenance of early Laramide sedimentary rocks in this area documented the presence of primarily volcanic and granitic source rocks. The uniformity of FT ages necessitates that source rocks, no matter their composition or age of deposition/crystallization, all record essentially the same thermal signatures. Accordingly, during the perhaps 10 m.y. of deposition of the Fort Crittenden Formation, the relative abundance of different rock units in the source terrane was rather uniform, and it is likely that there was no preferential exhumation of one source area over another. 2
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School:The University of Texas at Austin

School Location:USA - Texas

Source Type:Master's Thesis

Keywords:geology structural zirconium arizona

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