DEFINITION OF CHUGACH MOUNTAINS, ALASKA

Southern Alaska is underlain

by a complex amalgam of accreted terranes, including the

Wrangellia superterrane (consisting of the peninsular,

Wrangellia, and Alexander terranes), and farther outboard,

the Chugach–Prince William superterrane. During much of

the Mesozoic, the two superterranes formed a magmatic arc

and accretionary wedge, respectively, above a circum-Pacific

subduction zone. The Border Ranges fault forms the boundary

between the Wrangellia and Chugach–Prince William

superterranes; it initiated as a subduction thrust but has been

reactivated in various places as a strike-slip or normal fault.

On the Kenai Peninsula the Chugach terrane contains two

major units. Farther inboard lies the McHugh complex, composed

mainly of basalt, chert, argillite, and graywacke, as well

as several large ultramafic massifs. Radiolarians from

McHugh cherts throughout south-central Alaska range in age

from Ladinian (middle Triassic) to Albian-Aptian (mid-Cretaceous).

The interval during which the McHugh complex

formed by subduction-accretion is not well known, but probably

spanned most of the Jurassic and Cretaceous. The

McHugh has been thrust seaward on the Eagle River/Chugach

Bay fault over a relatively coherent tract of trench turbidites

assigned to the Upper Cretaceous Valdez Group. After the

protracted episode of subduction-accretion that built the

Chugach terrane, the accretionary wedge was cut by neartrench

intrusive rocks, assigned to the Sanak-Baranof plutonic

belt, probably related to ridge subduction.

The McHugh complex of south-central Alaska and its

lateral equivalent, the Uyak complex of Kodiak, are part of

the Mesozoic/Cenozoic subduction complex of the Chugach

terrane. Despite its vast extent and its potential value in

reconstructing the tectonics of the Pacific realm, the McHugh

has not been very extensively studied, especially compared to

similar tracts such as the Franciscan of California or the Shimanto

of Japan. The evolution of the McHugh and its equivalents

can be broken down into three broad, somewhat

overlapping phases: (1) origin of igneous and sedimentary

protoliths; (2) incorporation into the subduction complex

(“accretion”), and attendant deformation and metamorphism;

and (3) younger deformations.

Few fossil ages have been reported from the McHugh

complex with the best paleontological control now available

from the Seldovia quadrangle. At several places, mostly in

Kachemak Bay, radiolarian chert depositionally overlies pillow

basalt. Precise radiolarian age calls show that the base of

the chert varies in age from Ladinan to Albian-Aptian. Other

chert sections, which are typically fault-bounded and have no

stratigraphic context, also range from Ladinian to Albian.

Graywacke depositionally overlying chert has yielded Early

Jurassic (Pleinsbachian) radiolarians. These ages are readily

explained by a stratigraphic model in which the McHugh

basalts were formed by seafloor spreading, the overlying

cherts were deposited on the ocean floor as it was conveyed

toward a trench, and the argillite and graywacke record

deposition in the trench, just prior to subduction-accretion.

The timing of the subduction-accretion is not well known but

probably spanned most of the Jurassic and Cretaceous.

Limestones within the McHugh complex are of two categories.

A limestone clast in McHugh conglomerate has yielded

conodonts with a possible age range of late Meramecian

to early Morrowan (Late Mississippian to Early Pennsylvanian).

This clast could have been shed from the Strelna formation

of the Wrangellia terrane. Most of the dated limestones,

however, are tectonic blocks—typically occurring as severely

extended strings of boudins—that have yielded Permian

fusulinids or conodonts. Both the fusulinids and conodonts

are of shallow-water, tropical, Tethyan affinity; the fusulinids

are quite distinct from those of Wrangellia. The limestone

blocks might represent the tops of seamounts that were

decapitated at the subduction zone. If so, some of the ocean

floor that was offscraped to form the McHugh complex must

have formed in the Paleozoic.

The seaward part of the Chugach terrane is underlain by

the Valdez group of Late Cretaceous (Campanian? to Maastrichtian)

age. In the Kenai Peninsula, it includes medium- and

thin-bedded graywacke turbidites, black argillite, and minor

pebble to cobble conglomerate. These strata were probably

deposited in a deep-sea trench and accreted shortly thereafter.

Most of the Valdez group consists of relatively coherent strata,

deformed into regional-scale tight to isoclinal folds, cut by

a slaty cleavage. The McHugh complex and Valdez group are

juxtaposed along a thrust, which in the area of Turnagain

Arm has been called the Eagle River fault, and on the Kenai

Peninsula is known as the Chugach Bay thrust. Beneath this

thrust is a mélange of partially to thoroughly disrupted Valdez

group turbidites. This monomict mélange, which is quite distinct

from the polymict mélanges of the McHugh complex,

can be traced for many kilometers in the footwall of the Eagle

River thrust and its along-strike equivalents.

In early Tertiary time, the Chugach accretionary wedge

was cut by near-trench intrusive rocks forming the Sanak-

Baranof plutonic belt. The near-trench magmatic pulse

migrated 1,367 miles (2,200 km) along the continental margin,

from about 65–63 Ma at Sanak Island in the west to

about 50 Ma at Baranof Island in the east. The Paleogene

near-trench magmatism was related to subduction of the

Kula-Farallon spreading center.

Mesozoic and Cenozoic rocks of the accretionary wedge

of south-central Alaska are cut by abundant late brittle faults.

Along Turnagain Arm near Anchorage, four sets of late faults

are present: a conjugate pair of east-northeast-striking dextral

and northwest-striking sinistral strike-slip faults; north-northeast-

striking thrusts; and less abundant west-northwest-striking

normal faults. All four fault sets are characterized by

quartz ± calcite ± chlorite fibrous slickenside surfaces and

appear to be approximately coeval. The thrust- and strikeslip

faults together resulted in subhorizontal shortening perpendicular

to strike, consistent with an accretionary wedge

setting. Motion on the normal faults resulted in strike-parallel

extension of uncertain tectonic significance. Some of the

late brittle faults host gold-quartz veins, which are the same

age as nearby near-trench intrusive rocks. By implication, the

brittle faulting and gold mineralization are probably related

to ridge subduction.

Scattered fault-bounded ultramafic-mafic complexes in

southern Alaska stretch 600 miles (1,000 km) from Kodiak

Island in the south to the Chugach Mountains in the north.

These generally consist of dunite +/- chromite, wehrlite,

clinopyroxenite, and websterite, which grade upward into gabbronorites.

These rocks are intruded by quartz diorite, tonalite,

and granodiorite. Because of general field and petrographic

similarities, these bodies are generally regarded as having a

similar origin and are named the Border Ranges ultramaficmafic

complex (BRUMC). The BRUMC includes six bodies on

Kodiak and Afognak Islands, plus several on the Kenai Peninsula

(including Red Mountain, parts of the Kachemak terrane

(now obsolete, but the ultramafic complex is renamed the Halibut

Cove ultramafic massif), and other smaller bodies. In the

northern Chugach Mountains the BRUMC includes the Eklutna,

Wolverine, Nelchina, and Tonsina complexes and the

Klanelneechena complex in the central Chugach Mountains.

Some models for the BRUMC suggest that all these bodies

represent cumulates formed at the base of an intraoceanic

arc sequence and were cogenetic with volcanic rocks now

preserved on the southern edge of the Wrangellian composite

terrane located in the Talkeetna Mountains. However, some

of the ultramafic massifs on the southern Kenai Peninsula are

probably not related to this arc but represent deep oceanic

material accreted in the trench. The ultramafic massifs on the

Kenai Peninsula appear to be part of a dismembered assemblage

that includes the ultramafic cumulates at the base, gabbroic-

basalt rocks in the center, and basalt-chert packages in

the upper structural slices. The ultramafic massifs may represent

pieces of an oceanic plate subducted beneath the

Chugach terrane, with fragments offscraped and accreted

during the subduction process. There are several possibilities

as to what the oceanic plate may have been, including “normal”

oceanic lithosphere, an oceanic plateau, or an immature

arc. Alternatively, the ultramafic/mafic massifs may represent

a forearc or “suprasubduction zone” ophiolite, formed seaward

of the incipient Talkeetna (Wrangellia) arc during a

period of forearc extension.

See also CONVERGENT PLATE MARGIN PROCESSES.

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