Southern Alaska has a long history of subduction,
accretion, and coastwise transport of terranes (Coney
et al., 1980; Monger et al., 1982; Plafker et al., 1994).
The Chugach-Prince William (CPW) terrane is about
2200 km long and extends through much of southern
Alaska (Plafker et al., 1994) (Fig. 1A). The inboard
Chugach terrane can be divided into two parts, a
mélange and sedimentary units that are Permian to
Early Cretaceous in age and a turbidite sequence that
is from the Upper Cretaceous (Plafker et al., 1994).
In the Prince William Sound area, the outboard
Prince William terrane is comprised of Paleocene to
Eocene turbidites and associated basaltic rocks of the
Orca Group (Davidson and Garver, 2017), and the
turbidites of the inboard Chugach terrane are known
as the Valdez Group. The turbidites are intruded by
the Sanak-Baranof Belt (SBB), a group of 63-47 Ma
plutons that are progressively younger to the east.
The Border Ranges fault system marks the northern
boundary of the CPW terrane, separating the Chugach
terrane from the Wrangellia composite terrane and
the Contact fault separates the Chugach and Prince
William terrane (Fig. 1; Plafker et al., 1994).
There are three ophiolite sequences in the Orca Group:
Knight Island (KI), Resurrection Peninsula (RP),
and Glacier Island (GI) (Fig. 1B). The KI ophiolite
contains a sequence of massive pillow basalts, sheeted
dikes, and a minor amount of ultramafic rocks (Tysdal
et al, 1977; Nelson and Nelson, 1992; Crowe et al.,
1992). The RP ophiolite is a typical ophiolite sequence
and has interbedded Paleocene turbidites (Davidson
and Garver, 2017). Paleomagnetic data gathered
from the RP ophiolite indicated a mean depositional
paleolatitude of 54° ± 7° which implies 13° ± 9° of
poleward displacement (Bol et al., 1992). These data
suggest that the RP ophiolite was translated northward
to its current position after being formed in the Pacific
Northwest, and thus the CPW terrane may have been
originally located at 48-49° north and at 50 Ma was
transferred 1100 km to the north by strike-slip faulting
(Cowan, 2003). However, an opposing hypothesis
suggests that the terrane has not experienced
significant displacement and formed in Alaska due to
a now-subducted Resurrection plate (Haeussler et al.,
2003).
KI and RP ophiolites have traditionally been assumed
to be oceanic crust that was tectonically emplaced
into the CPW terrane (Bol et al., 1992; Lytwyn et
al., 1997). However, a more recent study suggests a
hypothesis that the ophiolites originated in an upper
plate setting and formed due to transtension (Davidson
and Garver, 2017). Previous workers have used
discriminant diagrams to identify the volcanic rocks
of KI ophiolite and RP ophiolite as mid-ocean ridge
basalts (Lytwyn et al., 1997; Miner, 2012). This project
presents new geochemical and geochronological data
from the GI ophiolite to determine its age and tectonic
setting. The purpose of this study is to compare the
data from GI with the data from KI and RP, and the
comparison of the geochemical data will allow for
a greater understanding of the tectonic setting of
southern Alaska.
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Geology of the Chugach-Prince William Terrane in northern Prince William Sound, Alaska.
This six-student project focused on the geology of
the Chugach and Prince William terranes in northern
Prince William Sound, Alaska. The Chugach-Prince
William (CPW) composite terrane is a Mesozoic-
Tertiary accretionary complex that is well exposed
for ~2200 km in southern Alaska and is inferred to
be one of the thickest accretionary complexes in the
world (Plafker et al., 1994; Cowan, 2003). The CPW
terrane is bounded to the north by the Border Ranges
fault, which shows abundant evidence of Tertiary
dextral strike slip faulting, and inboard terranes of the
Wrangellia composite terrane (Peninsular, Wrangellia,
Alexander) (Pavlis, 1982; Cowan, 2003; Roeske et
al., 2003). Throughout much of the 2200 km long
belt of the CPW terrane it is bounded by the offshore
modern accretionary complex of the Alaskan margin,
but east of Prince William Sound the Yakutat block is
colliding into the CPW and this young collision has
significantly affected uplift and exhumation of inboard
rocks.
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- Award ID(s):
- 1728013
- NSF-PAR ID:
- 10181882
- Date Published:
- Journal Name:
- Proceedings of the Keck Geology Consortium.
- Volume:
- 32
- Page Range / eLocation ID:
- 1-5
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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