Abstract
The Alaska Peninsula is composed of the late Paleozoic
to Quaternary sedimentary, igneous, and minor metamorphic
rocks that record the history of a number of magmatic arcs.
These magmatic arcs include an unnamed Late Triassic(?)
and Early Jurassic island arc, the early Cenozoic Meshik arc,
and the late Cenozoic Aleutian arc. Also found on the Alaska
Peninsula is one of the most complete nonmetamorphosed,
fossiliferous, marine Jurassic sedimentary sections known. As
much as 8,500 m of section of Mesozoic sedimentary rocks
record the growth and erosion of the Early Jurassic island arc.
A thinner, but still thick (as much as 5,400 m), sequence of
Tertiary sedimentary rocks that are predominantly continental
overlies the Mesozoic section. A brief regression in early Tertiary
time on the Alaska Peninsula and granodiorite plutonism in the
Shumagin, Semidi, and Sanak Islands was followed by deposition of fluvial and minor marine clastic strata. This was followed by deposition of transgressive marine clastic strata and initiation
of the Meshik arc, shown by an areally extensive outpouring of
volcanic and volcaniclastic rocks and debris between late Eocene
and earliest Miocene time. Late Miocene time was marked by
another brief transgression and northwest- to southeast-directed
compression, followed by renewed volcanism and plutonism
which initiated the modern Aleutian magmatic arc.
Extensive glacial and glaciomarine deposits of late
Pleistocene age create an extensive lowland physiographic
province on the northwest side of the Alaska Peninsula and
join isolated mountain masses to the Alaska Peninsula on
the southwest. Multiple active volcanoes and volcanic peaks
dominate the skyline of the Alaska Peninsula and represent the
continuation of magmatic activity that has formed the Aleutian
arc since late Miocene time.
The Alaska Peninsula has had a long and involved history since Paleozoic time. We propose that the Paleozoic and
Mesozoic rocks that constitute much of the Alaska Peninsula
be called the Alaska Peninsula terrane. Using the concept of
subterranes, we divide the terrane into two distinct but tectonically related subterranes: the Chignik and Iliamna subterranes,
which share a limited common geologic history. The Iliamna
subterrane has served at most times as a source area for the
Chignik subterrane; however, some rock units are in common across the subterranes. The Iliamna and Chignik subterranes are in part separated by the Bruin Bay fault system.
The Iliamna subterrane is composed of moderately deformed
early Mesozoic marine sedimentary and volcanic rocks and
schist, gneiss, and marble of Paleozoic(?) and Mesozoic age,
and plutonic rocks of the Alaska-Aleutian Range batholith.
Characteristic of the Chignik subterrane are little-deformed,
shallow-marine to continental clastic sedimentary rocks ranging in age from Permian to latest Cretaceous. However, deep-marine, volcaniclastic, and calcareous rocks form important
components of the older rocks in the subterrane.
The two subterranes of the Alaska Peninsula terrane are
characterized by radically different structural and metamorphic styles. The nonplutonic rocks of the Iliamna subterrane
are characterized by metamorphism up to amphibolite-facies
grade and intense folding. In the Chignik subterrane, the structural style is dominated by large, open, en echelon anticlinal
structures, normal faulting, and thrust and high-angle reverse
faults that have minor displacement in a northwest to southeast
direction. In the Outer Shumagin and Sanak Islands, rocks
assigned to the Chugach terrane are characterized structurally by tight, generally northeast-trending folds. Dips in these
rocks tend to be steep, rarely less than 35°, and overturned
beds are locally common.
The boundaries separating the Alaska Peninsula terrane from other terranes are commonly indistinct or poorly
defined. A few boundaries have been defined at major faults,
although the extensions of these faults are speculative through
some areas. The west side of the Alaska Peninsula terrane is
overlapped by Tertiary sedimentary and volcanic rocks and
Quaternary deposits.