Geology
of the Talkeetna Volcanic Formation
Peninsula Terrane, south
central Alaska
View of Nelchina Glacier and River cutting through the
entire arc crust section, south of the Matanuska Valley.
The Scientific
Question
Magmatism at convergent plate
margins represents the second largest source of new crust to the Earth’s
surface after the mid ocean ridge system, but unlike the oceanic lithosphere,
which is almost completely recycled back into the upper mantle, arc rocks
may be incorporated into the continental crust and preserved. As a
result, it has been argued that it is in such settings that the continental
crust was generated. Controversy over this hypothesis continues because island
arc crust is typically too mafic and too light rare earth element depleted
to form continental crust by itself. While some workers have argued that arc
crust is transformed into continental crust during final collision with a
passive continental margin, serious problems
remain in understanding the composition of the original arc crust.
Camp site in the East Boulder Creek. Monarch Mount comprises a
late granite penetrating
lavas, ignimbrites and debris flows from the upper part of the Talkeetna
Volcanic Formation.
The Study Area
The Talkeetna Arc is located in south central Alaska and forms part of
the Peninsular Terrane, one of a series of allochthonous tectonic units
accreted to the active margin of North America during the Mesozoic and Cenozoic.
The Peninsular Terrane is believed to has amalgamated with two other oceanic
fragments, the Alexander and Wrangellia Terranes, prior to their accretion
during the latest Jurassic-Early Cretaceous to North America, as the Wrangellia Composite Terrane. Subsequent northward
subduction under Alaska has juxtaposed a large clastic subduction accretionary
wedge along the southern edge of the Peninsular Terrane, comprising the
Chugach Terrane in the region of the study.
The Chugach Terrane is separated from the Talkeetna Arc by the Border Ranges
Fault, which is a major right lateral strike-slip fault.
The Talkeetna arc section, especially in the Nelchina-Tonsina
region, was first recognized by Burns (1983, 1985) and DeBari and Coleman
(1989). Isolated exposures of high pressure ultramafic and mafic
rocks associated with the arc lie along the southern boundary and have
been dubbed the “Border Ranges Ultramafic and Mafic Complex”. Associated
moderate pressure mafic plutonic rocks comprise a continuous belt at least
120 km long and as much as 10 km wide from Tonsina in the east to at least
the Matanuska Glacier in the west.
Age control on the Talkeetna Arc is presently sparse.
40Ar-39Ar ages of 177–181 Ma (Aalenian-Toarcian) were measured in gabbros
from the mafic plutonic belt and 180–182 Ma for the high pressure gabbro
of the Border Ranges Ultramafic and Mafic Complex (Onstott et al., 1989).
In the Talkeetna Volcanic Formation there is some biostratigraphic control.
The base of the Talkeetna Volcanic Formation is exposed in the Alaska Peninsular
and has been dated as Hettangian (Lower Jurassic, 198 Ma; Pálfy
et al., 1999). The top of the Talkeetna Volcanic Formation is overlain
by the Tuxedni Formation, dated as Middle Jurassic on the basis of Early
Bajocian molluscs found in the lower part of the formation (~172 Ma).
Satellite image (NASA)
of south central Alaska, showing the study area along the
northern edge of the Chugach Mountains and in the Talkeetna Mountains,
south and west of the Copper River Basin.
Summary of Major Results to date
The Early Jurassic Talkeetna Volcanic Formation forms the upper stratigraphic
level of an oceanic volcanic arc complex within the Peninsular Terrane
of south central Alaska. The arc section dips north (Figure 1) from its contact along the Border Ranges
Fault, with the Cretaceous Chugach Terrane, an accretionary complex. The
section comprises a series of lavas, tuffs, ignimbrites, as well as volcaniclastic
debris flow and turbidite deposits, showing significant lateral facies
variability. There is a general trend to more volcaniclastic sediment at
the top of the section (Horn Mountains) and more
lavas and ignimbrites towards the base. Mapping allows a generalized composite section to be constructed. Evidence
for both subaerial and submarine emplacement is seen through the section,
which totals approximately 7 km, similar to modern western Pacific Arcs.
Chemically the Talkeetna Volcanic Formation is tholeiitic (Figure 2) and shows no clear trend to increasing SiO
2 up-section. A subduction petrogenesis
is clear in multi-element “spider diagrams” (Figure
3). Rocks at the base of the section exposed in the Stuck and Willow
Mountain Massifs and south of the Matanuska River show no relative
enrichment in light rare earth element (LREE) and high field strength elements
(HFSEs), but become slightly enriched up-section (Sheep Mountain Massif; Figure 4). The Talkeetna Volcanic Formation is typically
more REE depleted than either the continental crust or the liquids inferred
to have reached the middle crust.
However, at Stuck Mountain (Tonsina District) at the base of the section,
small volumes of mafic REE enriched lavas are recognized (Figure 5). The Talkeetna Volcanic Formation does not
show evidence for the recycling of significant volumes of continental crust,
implying that it was formed above a north-dipping subduction zone, with
no record of its collision with North America. The degree of LREE and HFSE
enrichment correlates most strongly with that seen in the Tonga and Izu Arc
systems and confirms that the Talkeetna Arc formed far from a continental
mass (Figure 6). The Talkeetna Volcanic Formation
is not a suitable candidate as a building block for the continental crust
without significant addition from more enriched sources.
Rainbow in
the Little Oshetna Valley where vescular basalts and volcanic
debris flow
conglomerates of the Talkeetna Volcanic Formation are overlain
by Middle Jurassic
clastic sedimentary rocks (Trop et al., 2002).
This study forms part of a larger project
to characterize the chemistry, timing and lithology of a complete oceanic
island arc crustal section exposed in the Talkeetna Arc of Alaska. Project
supported by the National Science Foundation (team leader Peter Kelemen). Intellectual
and logistical help in the field in 2002 thanks to Amy Draut and Karen Hanghoj.
Additonal help came from fellow investigators:
Greg Hirth and Stan Hart (Woods Hole Oceanographic Institution)
Brad Hacker, Jim Mattinson, Luc Mehl and Matt Rioux (University of California,
Santa Barbara)
Terry Pavlis (University of New Orleans)
Sue DeBari and Andrew Greene (Western Washington University)
Nikolas I. Christensen (Wisconsin)
Return to home page.
Page editor
Peter Clift
Last modified 10/1/2002
