A HISTORY OF GEOLOGIC RESEARCH ON SEWARD PENINSULA, ALASKA, BY ELIZABETH MILLER AND COLLABORATORS: CRUSTAL EXTENSION FROM TOP TO BOTTOM

In 1987, Elizabeth Miller, on a field trip with ARCO, visited blueschist to granulite facies rocks on the Seward Peninsula (SP) previously studied by Evans and students at U. Washington. From ‘89–‘95, Miller’s students and collaborators worked on high-T rocks and plutons of the Kigluaik Mtns gneiss dome and on high-P/low-T rocks of the Nome lowlands. Eventually the entire range was mapped, encompassing >1000 km². Over a dozen publications resulted, documenting high-T fabrics whose stretching direction switched from N-S at high levels to E-W flow at depth, converting plutons as young as 105 Ma to gneisses cut by by undeformed plutonic rocks (90–85 Ma) and diabase dikes (84–80 Ma). Since this work, we have studied other areas of SP, including the Bendeleben, Darby and York Mtns. A picture emerges of extensional processes from brittle deformation in the upper crust to granulite facies crustal flow at depth. The York Mtns. expose the upper levels of the extensional system where a detachment fault separates rocks with subhorizontal greenschist fabrics in the lower plate from normal faulted Ordovician-Silurian carbonates in the upper plate. Extension was N-S and took place prior to 78 Ma cross-cutting dikes. The Bendeleben Mtns. consist mainly of high-grade rocks cut by deformed and undeformed Cretaceous plutons. Neoproterozoic (870 Ma) and Devonian (380 Ma) felsic gneisses are present. Stretching lineations are poorly defined and deformation occurred in a mostly pure shear environment during emplacement of the 104 Ma S-type Bendeleben pluton. The Darby Mtns. expose deformed granites of Devonian (390 Ma) age cut by the undeformed Darby pluton (~100 Ma). The Kugruk shear zone, east of the Darbys, is a 6-km- vertical right-lateral shear zone. Its motion may have occurred during or after fabrics formed that yield 130 Ma ages (⁴⁰Ar/³⁹Ar). The shear zone, with crossite-bearing metabasites and serpentinite, is parallel to the mapped boundary between the Arctic Alaska and Koyukuk terrane and may have been active during opening of the Canada Basin. Extension along regional normal faults in the mid Cenozoic created the modern topography and adjacent sedimentary basins of the SP with historic earthquakes on some faults.