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ABSTRACT: Constraints by Apatite Fission-Track Analysis on Cooling Times Due to Uplift in the Northeastern Brooks Range, Alaska

Paul B. O'Sullivan, John Decker, Steven C. Bergman, Wesley Wallace

This report describes application of apatite fission track analysis (AFTA) on Devonian igneous rocks and Mesozoic and Tertiary sedimentary rocks exposed in the northeastern Brooks Range of northeastern Alaska. The AFTA data helps constrain the regional tectonic and thermal history of the area.

New data are available from four locations: the Okpilak batholith, the Jago River section east of the Sadlerochit Mountains, the Marmot syncline, and the Sagwon Bluffs region. AFTA data on rocks from both a vertical and a horizontal traverse of the Devonian-aged Okpilak batholith (n = 17) indicate two distinct cooling events. The first resulted in rapid (<3-5 m.y.) cooling through 125-60°C at ^sim40-45 Ma. The presence of the top of a fossil annealing zone near the base of the vertical section indicates a maximum age of the second cooling at 30 Ma. AFTA data on sedimentary rocks from the Paleocene to Eocene Jago River Formation (n = 5) indicate rapid cooling (<3-5 m.y.) at approximately 20 Ma. AFTA data on Albian-aged sedimentary rocks exposed at Marmot syncline (n = S) ind cate cooling at ^sim40-45 Ma. Data from Early Tertiary sedimentary rocks (n = 4) exposed at Sagwon Bluffs along the Sadlerochit River show the region has not been heated above 60°C since deposition, and the individual grain ages (< 120 Ma) reflect the thermal histories of the provenance terrains.

The northeastern Brooks Range is structurally complex because folding and thrust faulting in the region extends farther to the north and involves younger sediments, compared with deformation throughout the remainder of the Brooks Range. These new data, combined with previous results which show major cooling due to uplift and erosion at 60 Ma at Bathtub Ridge and at 45 Ma along the Canning River west of the Sadlerochit Mountains, now define three major phases of cooling due to uplift and erosion in northeastern Alaska at approximately 60 Ma, ^sim40-45 Ma, and approximately 20 Ma.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990