MESOZOIC AND TERTIARY STRUCTURAL HISTORY OF THE NORTHERN TALKEETNA MOUNTAINS

New geologic mapping along the Talkeetna Mountains Transect, combined with new gravity, magnetic and magnetotelluric data models, constrain the orientation, offset and dip of structures in the northern Talkeetna Mountains. Dominantly contractional structures responsible for the Jurassic to early Cretaceous accretion of Wrangellia, were re-activated in Tertiary time with reverse, extensional and strike-slip motion. Steep, prominent N30E faults have motion of at least Paleocene to Miocene age; some are the locus of current shallow seismicity.

Inboard of Wrangellia, reverse faults offset rocks at least as old as Late Triassic. Low-angle faults, which duplicate Paleocene granites, are cut by later steep, probable reverse faults that uplift the granites and coeval volcanic rocks. Cooling ages of intrusions and gneissic wallrocks suggest early rapid uplift, with a probable component of dextral strike-slip motion on N30E structures. En-echelon segmentation along the previously recognized Susitna Lineament also suggests dextral motion. A composite Middle Jurassic batholith that intruded the outboard edge of Wrangellia was uplifted at least 2-4 km before Late Jurassic time by the near-vertical Tsisi Creek Fault.

The Fog Lakes lowland developed above a deep (>2km) crustal break separating dense, magnetic oceanic crust (to the East) from lighter, non-magnetic transitional crust to the West. That crustal break, typically called the "leading edge" of Wrangellia, dips steeply westward, and is a distinctly different structure from the shallow-east-dipping Talkeetna Thrust as previously depicted. Numerous faults occupy a broad 10km-wide zone above the crustal break. The Fog Lakes lowland is a trapezoidal basin floored by Jurassic to Cretaceous meta-flysch and filled by the Eocene Deadman volcanic field. Its eastern bounding structure is a steep west-dipping N30E extensional fault bounding the Watana Creek Oligocene to Miocene basin. The lowland extends southward to colinear structures along Stephan and Portage Lakes. Its orientation suggests development during dextral motion.

Our new mapping suggests a structurally complex, but related history of crustal blocks from the outboard edge of Wrangellia to the flysch basin being deposited along its leading edge during accretion to the North American margin.