A Preliminary Low-Temperature Exhumation Study of the Central Aleutian
Islands, Alaska
Abstract
Exhumation in continental and oceanic arc settings is linked to tectonic
and climatic forces that result in some of the highest topography and
erosion rates in the world. Regional exhumation studies of oceanic arcs
are sparse due to poor exposure, accessibility, complex inherited
structures, and thermal overprinting due to magmatism, reburial, and
metamorphism. In contrast, the Aleutian arc has an unusually thick
crust, exceptional subaerial exposure of plutons, and limited regional
thermal overprinting, providing the ideal conditions for a systematic
thermochronology study that investigates the mechanisms that lead to arc
exhumation. By analyzing multiple chronometers (apatite and zircon
(U-Th)/He, zircon U-Pb) with different temperature sensitivities within
the same plutonic sample, we can constrain uplift and erosion rates over
~40 million years of Aleutian arc history. Here, we
present preliminary apatite and zircon (U-Th)/He ages from 23 plutonic
samples from the islands of Unalaska, Umnak, Atka, Ilak, and Amatignak.
These data are part of a larger study where we will analyze a total of
78 samples collected from 11 plutons along >1400 km of the
Central Aleutian Islands. Ultimately, this dataset will provide a
regional framework to quantitatively assess the various proposed
mechanisms for Aleutian arc exhumation, such as; 10° Pacific plate
rotation, subduction of the Kula ridge, and/or pluton emplacement, each
of which has predictable and testable geographic age trends. The samples
analyzed are from plutons that crystallized in the Oligocene to Miocene
based on zircon U-Pb dating. Both zircon and apatite (U-Th)/He ages also
show an Oligocene to Miocene spread, with a majority of the ages from
both chronometers pooling at 7-13 Ma and no particular geographic trend.
These preliminary results suggest that a significant exhumation event
occurred in the late Miocene, as has been observed in other
circum-Pacific arcs. These preliminary data may support the hypothesis
that the late Miocene pulse of high plutonism in the Aleutian Islands
led to arc exhumation. However, additional samples need to be analyzed
to provide an arc-scale view of exhumation timing, trends, and rates of
the Central Aleutian arc in order to test possible uplift and erosion
scenarios.