CONSTRAINTS ON LOCATING THE PROTO-DENALI FAULT IN SOUTHEAST ALASKA

The Denali Fault (DF) is a crustal scale strike-slip fault that extends >2000 km from north central to southeast Alaska (SEAK). The DF postdates initial terrane accretion, marks 10-15 km contrasts in Moho depth, and has >400 km dextral separation since the mid-Cretaceous. In SEAK northwest-trending orogen-parallel faults have accommodated oblique transpression between the North American margin, accreted terranes, and oceanic plates since Jurassic time. The DF has diffuse seismicity west of its convergence with the Chatham Strait Fault (CSF); seismicity on or east of the CSF is lacking. The CSF and major structures east of the CSF, including the Coast shear zone (CSZ), Kensington megashear (KM), and Gastineau Channel Fault (GCF) are candidates for a proto-DF segment in SEAK that is no longer active.

The CSZ is 2-6 km wide and extends >800 km in SEAK and coastal British Columbia. It contains quartz-filled plastic structures 2-20 m wide that cut mid-Cretaceous contractional deformation and 65 Ma plutons; it predates a 58 Ma dike. Kinematic indicators support west side up separation at 84-65 Ma and east side up separation at 65-50 Ma. Seismic imaging shows contrasting Moho depths of ~30 km east of the CSZ and ~25 km to the west. The CSZ both follows and crosses terrane boundaries but lacks tie points. In northern SEAK the Alaska Juneau (AJ) gold deposit formed in CSZ shear structures within the Insular terrane. Biotite from ore-bearing diorite yielded Ar-Ar dates of 57-58 Ma; white mica in mineralized structures is 56 Ma. The KM cuts the Insular terrane 3 km west of the CSZ and hosts the Kensington gold deposit, consisting of mesothermal gold-bearing quartz veins in steep shear zones locally >350 m wide. White mica in mineralized structures yielded Ar-Ar dates of 53-56 Ma, suggesting KM is a splay of the CSZ.

The GCF, 3 km west of the KM, juxtaposes rocks of different age and metamorphic grade. West of the GCF white mica in gold-mineralized structures at the Treadwell mine yielded Ar-Ar dates of 53-56 Ma, suggesting Treadwell and AJ structures define a ~6 km-wide CSZ corridor that includes KM and GCF.

The CSF truncates the GCF and 25 Ma granite and has inferred dextral separation of ~180 km. The trace of KM aligns with the mapped trace of DF at its convergence with CSF at the head of Chilkat Inlet, suggesting transition of DF activity from CSZ to CSF after 25 Ma.