GEOLOGIC MAPPING AND STRUCTURAL ANALYSIS TO DETERMINE THE EVOLUTION OF THE DARBY MOUNTAINS REGION AND KUGRUK FAULT ZONE, SEWARD PENINSULA, ALASKA

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.).