Evolving heavy mineral assemblages reveal changing exhumation and trench tectonics in the Mesozoic Chugach accretionary complex, south-central Alaska

Peter Clift, Nico Monique Wares, Jeffrey M. Amato, Terry L. Pavlis, Malcolm John Hole, Caleb Worthman, Erik Day

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Abstract

The Gulf of Alaska is one of the largest accretionary complexes on Earth. In this study, we examined the earliest phase of accretion in the Mesozoic McHugh Complex and Valdez Groups, exposed in SE Alaska. The oldest preserved fragment, the Mesomélange assemblage, is Jurassic (ca. 160–140 Ma) and consists of an ~3-km-thick structural package of strongly deformed shaley materials with slices of oceanic cherts and basalts. Heavy minerals indicate dominant erosion from a magmatic arc source uplifted after the collision of the Wrangellia and the Talkeetna oceanic arc. A tectonic erosion event affected the forearc just prior to ca. 120 Ma and was likely caused by seamount collision, ridge subduction, or both. This was followed at 105 Ma by mass wasting of sandstone and conglomerates, preserved as the Graywacke-Conglomerate assemblage (ca. 105–83 Ma). Heavy minerals indicate continued flux from arc sources, but with significant changes suggesting a larger, more diverse catchment area. Erosion of deeper crustal sources provided high-Mg diopside and garnets to the trench. Faster sediment fl ux was caused by rock uplift triggered by final accretion of the Wrangellia-Peninsula terrane to North America. The start of large-scale accretion in Alaska roughly coincided with the initiation of Shimanto Complex accretion in Japan and can be understood as primarily linked to sediment supply driven by plate-margin tectonics rather than climatically induced erosion onshore.
Original languageEnglish
Pages (from-to)989-1006
Number of pages18
JournalGeological Society of America Bulletin
Volume124
Issue number5-6
Early online date29 Nov 2011
DOIs
Publication statusPublished - May 2012

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heavy mineral
exhumation
trench
accretion
erosion
tectonics
conglomerate
collision
mass wasting
graywacke
diopside
seamount
sediment
terrane
garnet
subduction
Jurassic
basalt
uplift
sandstone

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Evolving heavy mineral assemblages reveal changing exhumation and trench tectonics in the Mesozoic Chugach accretionary complex, south-central Alaska. / Clift, Peter; Wares, Nico Monique; Amato, Jeffrey M. ; Pavlis, Terry L. ; Hole, Malcolm John; Worthman, Caleb ; Day, Erik .

In: Geological Society of America Bulletin, Vol. 124, No. 5-6, 05.2012, p. 989-1006.

Research output: Contribution to journalArticle

Clift, Peter ; Wares, Nico Monique ; Amato, Jeffrey M. ; Pavlis, Terry L. ; Hole, Malcolm John ; Worthman, Caleb ; Day, Erik . / Evolving heavy mineral assemblages reveal changing exhumation and trench tectonics in the Mesozoic Chugach accretionary complex, south-central Alaska. In: Geological Society of America Bulletin. 2012 ; Vol. 124, No. 5-6. pp. 989-1006.
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title = "Evolving heavy mineral assemblages reveal changing exhumation and trench tectonics in the Mesozoic Chugach accretionary complex, south-central Alaska",
abstract = "The Gulf of Alaska is one of the largest accretionary complexes on Earth. In this study, we examined the earliest phase of accretion in the Mesozoic McHugh Complex and Valdez Groups, exposed in SE Alaska. The oldest preserved fragment, the Mesom{\'e}lange assemblage, is Jurassic (ca. 160–140 Ma) and consists of an ~3-km-thick structural package of strongly deformed shaley materials with slices of oceanic cherts and basalts. Heavy minerals indicate dominant erosion from a magmatic arc source uplifted after the collision of the Wrangellia and the Talkeetna oceanic arc. A tectonic erosion event affected the forearc just prior to ca. 120 Ma and was likely caused by seamount collision, ridge subduction, or both. This was followed at 105 Ma by mass wasting of sandstone and conglomerates, preserved as the Graywacke-Conglomerate assemblage (ca. 105–83 Ma). Heavy minerals indicate continued flux from arc sources, but with significant changes suggesting a larger, more diverse catchment area. Erosion of deeper crustal sources provided high-Mg diopside and garnets to the trench. Faster sediment fl ux was caused by rock uplift triggered by final accretion of the Wrangellia-Peninsula terrane to North America. The start of large-scale accretion in Alaska roughly coincided with the initiation of Shimanto Complex accretion in Japan and can be understood as primarily linked to sediment supply driven by plate-margin tectonics rather than climatically induced erosion onshore.",
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note = "ACKNOWLEDGMENTS This work was funded by National Science Foundation grants EAR-0809608 to Amato and EAR-0809609 to Pavlis. Clift and Wares thank Nilanjan Chatterjee from the Massachusetts Institute of Technology for his help with the electron probe analyses and John Still from University of Aberdeen for help with the scanning electron microscope and advice on the interpretation of mineral data. Jeremy Preston provided software for calculation of mineral formulas from probe data from http://www.gabbrosoft.org/. Marc Robertson provided important assistance at an early stage in this project. Joe Hecker (New Mexico State University) helped with some of the mineral separations. The paper benefited from helpful reviews from Peter Cawood, David Scholl, and an anonymous reviewer.",
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AU - Amato, Jeffrey M.

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AU - Hole, Malcolm John

AU - Worthman, Caleb

AU - Day, Erik

N1 - ACKNOWLEDGMENTS This work was funded by National Science Foundation grants EAR-0809608 to Amato and EAR-0809609 to Pavlis. Clift and Wares thank Nilanjan Chatterjee from the Massachusetts Institute of Technology for his help with the electron probe analyses and John Still from University of Aberdeen for help with the scanning electron microscope and advice on the interpretation of mineral data. Jeremy Preston provided software for calculation of mineral formulas from probe data from http://www.gabbrosoft.org/. Marc Robertson provided important assistance at an early stage in this project. Joe Hecker (New Mexico State University) helped with some of the mineral separations. The paper benefited from helpful reviews from Peter Cawood, David Scholl, and an anonymous reviewer.

PY - 2012/5

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N2 - The Gulf of Alaska is one of the largest accretionary complexes on Earth. In this study, we examined the earliest phase of accretion in the Mesozoic McHugh Complex and Valdez Groups, exposed in SE Alaska. The oldest preserved fragment, the Mesomélange assemblage, is Jurassic (ca. 160–140 Ma) and consists of an ~3-km-thick structural package of strongly deformed shaley materials with slices of oceanic cherts and basalts. Heavy minerals indicate dominant erosion from a magmatic arc source uplifted after the collision of the Wrangellia and the Talkeetna oceanic arc. A tectonic erosion event affected the forearc just prior to ca. 120 Ma and was likely caused by seamount collision, ridge subduction, or both. This was followed at 105 Ma by mass wasting of sandstone and conglomerates, preserved as the Graywacke-Conglomerate assemblage (ca. 105–83 Ma). Heavy minerals indicate continued flux from arc sources, but with significant changes suggesting a larger, more diverse catchment area. Erosion of deeper crustal sources provided high-Mg diopside and garnets to the trench. Faster sediment fl ux was caused by rock uplift triggered by final accretion of the Wrangellia-Peninsula terrane to North America. The start of large-scale accretion in Alaska roughly coincided with the initiation of Shimanto Complex accretion in Japan and can be understood as primarily linked to sediment supply driven by plate-margin tectonics rather than climatically induced erosion onshore.

AB - The Gulf of Alaska is one of the largest accretionary complexes on Earth. In this study, we examined the earliest phase of accretion in the Mesozoic McHugh Complex and Valdez Groups, exposed in SE Alaska. The oldest preserved fragment, the Mesomélange assemblage, is Jurassic (ca. 160–140 Ma) and consists of an ~3-km-thick structural package of strongly deformed shaley materials with slices of oceanic cherts and basalts. Heavy minerals indicate dominant erosion from a magmatic arc source uplifted after the collision of the Wrangellia and the Talkeetna oceanic arc. A tectonic erosion event affected the forearc just prior to ca. 120 Ma and was likely caused by seamount collision, ridge subduction, or both. This was followed at 105 Ma by mass wasting of sandstone and conglomerates, preserved as the Graywacke-Conglomerate assemblage (ca. 105–83 Ma). Heavy minerals indicate continued flux from arc sources, but with significant changes suggesting a larger, more diverse catchment area. Erosion of deeper crustal sources provided high-Mg diopside and garnets to the trench. Faster sediment fl ux was caused by rock uplift triggered by final accretion of the Wrangellia-Peninsula terrane to North America. The start of large-scale accretion in Alaska roughly coincided with the initiation of Shimanto Complex accretion in Japan and can be understood as primarily linked to sediment supply driven by plate-margin tectonics rather than climatically induced erosion onshore.

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JO - Geological Society of America Bulletin

JF - Geological Society of America Bulletin

SN - 0016-7606

IS - 5-6

ER -