GEOLOGIC MAPPING AND STRUCTURAL ANALYSIS TO DETERMINE THE EVOLUTION OF THE
DARBY MOUNTAINS REGION AND KUGRUK FAULT
ZONE, SEWARD PENINSULA, ALASKA
ROMERO, Geovanni, Geological Sciences, New Mexico State University, MSC 3AB, PO Box 30001, Las Cruces, NM 88003, [email protected], AMATO, Jeffrey, Geological Sciences, New Mexico State Univ, MSC 3AB, PO Box 30001, Las Cruces, NM 88003, and TORO, Jaime, Department of Geology and Geography, West Virginia Univ, 425 White Hall, P.O. Box 6300, Morgantown, WV 26506-6300
Geologic mapping at 1:24,000 scale was carried out in the southeastern
Bendeleben and northeastern Solomon quadrangles of eastern Seward Peninsula
where the N-S trending Darby Mountains lie adjacent to the Kugruk fault
zone.
This
fault
is thought to have played a fundamental role in the exhumation of the
metamorphic rocks of Seward Peninsula adjacent to the Yukon-Koyukuk basin, yet
little is known about this enigmatic structure. The Kugruk
fault
zone strikes
north-south and is over 100 km long and ~10 km wide. The stratigraphic units
mapped in this area include from east to west: The rocks of the
fault
zone:
Cretaceous-Tertiary carbonate sandstone and conglomerate; mylonitic metabasite,
serpentine, and gabbro of undetermined age, and marble. Low-grade rocks west of
the
fault
zone: low-grade metamorphosed and deformed quartzite, calc-schist, and
marble, dolostone with Paleozoic protoliths corresponding to the Nome Group; the
Cretaceous Darby pluton; and deformed upper-greenschist to lower-amphibolite
grade metamorphic rocks correlative to those mapped to the west in the Kigluaik
Mountains. Tertiary and Quaternary volcanic rocks and unconsolidated deposits
are also present.
The metabasic rocks have blue amphibole (crossite). Metamorphic grade in the
rocks west of the fault
increases from east to west. White-mica and
chlorite-albite schists are common to the east, whereas biotite, sillimanite,
and garnet are present in schists to the west. Foliation in metamorphic units
adjacent to the
fault
is well-defined, striking consistently north-northeast
with variable but steep dips generally to the west. Field-based analysis of
kinematic indicators such as sigma-clasts and displacement along shear bands
suggest a right-lateral sense of motion along this
fault
. Four oriented samples
from within and immediately west of the
fault
zone have steep dips of foliation,
subhorizontal lineations, and right-lateral sense of shear along the N-S zone.
Lineations in the higher grade rocks are steeper, with one sample showing a
reverse shear sense. Younger east-west striking faults cut the units near the
fault
zone, including the metabasic rocks. A 2005 earthquake in the region was
determined to be on an east-west normal
fault
(Natalia Ruppert, pers. comm.).