Special Seminar: |
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Dr. Brian Hampton, Michigan State University
"A detrital record of arc accretion, exhumation, and basin development in the North American Cordillera, southern Alaska"
Wednesday, 10 September
12 noon - 12:55pm
Dillman 320
A reception with snack will follow in the Dow 6th Floor Atrium
Please join us to learn about Alaskan tectonics, rather than Alaskan politics! Instead of scouring the detritus of political machinations for insight into current events, we will see the history of Alaskan tectonic movement from its own detritus, and the record it leaves as well as the geologic insight it provides.
ABSTRACT:
The detrital record of Mesozoic island arc accretion along the North
American Cordillera is preserved in a discontinuous belt of clastic marine
strata that are exposed inboard of the Wrangellia island arc from
southwestern Alaska to Washington State. In southern Alaska, synorogenic
deposits of the Upper Jurassic-Cretaceous Kahiltna assemblage contain the
stratigraphic record of Wrangellia island arc accretion. U-Pb detrital
zircon ages combined with detailed stratigraphic constraint from the
Kahiltna assemblage reveal a distinct temporal and spatial trend in regional
exhumation and basin-fill evolution during Jurassic-Cretaceous time.
LA-ICPMS analysis of detrital zircon grains from a >250-km-long transect of
the Kahiltna basin reveal a U-Pb age distribution of primarily Mesozoic age
grains (74%) with less abundant Paleozoic (11%), and Precambrian (15%) age
grains. Preexisting biostratigraphic constraint from the Kahiltna basin
together with a minimum crystallization age (maximum depositional age) from
each of the 8 samples allows for the study of upsection changes in
provenance within the Kahiltna assemblage, which, ultimately reflect a
distinct pattern of exhumation and erosion along the margins of the basin
during island arc accretion. A comparison of detrital zircon age
distribution through time in the Kahiltna assemblage reveals three distinct
stages of exhumation and basin development that include: (1) An initial Late
Jurassic (latest Oxfordian) stage during which detrital zircon grains were
derived solely from Late Triassic-Late Jurassic igneous sources of the
oceanic Wrangellia composite terrane (Mz-100%-Pz-0%-Pc-0%), (2) an Early
Cretaceous (Barremian-Aptian) stage during which detritus was derived
primarily from Precambrian, Devonian-Mississippian, and Late
Triassic-Jurassic source areas of the continental margin, as well as Late
Triassic-Cretaceous arc sources of the oceanic Wrangellia composite terrane
(Mz-76%-Pz-12%-Pc-12%); and, (3) an Early Cretaceous (Albian) stage that is
represented by zircon age populations that reflect a large component of
Precambrian and Ordovician-Silurian sources of the continental margin, as
well as Cretaceous sources of the Wrangellia composite terrane
(Mz-19%-Pz-22%-Pc-59%). A comparison of along-strike detrital trends from
throughout southern Alaska are consistent with a model favoring northward
progression of exhumation, erosion, and deposition in a series of
syncollisional basins during oblique collision of the oceanic Wrangellia
composite terrane to the Cordilleran continental margin during Late
Jurassic-Early Cretaceous time. |
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For more information about this talk , please contact the Department Geological and Mining Engineering and Sciences—487-2531
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9/08/2008 |
