A FAULT ROCK RECORD OF LATE EOCENE STRAIN PARTITIONING ALONG THE DENALI FAULT ZONE IN SOUTHWESTERN YUKON

Several hundred m SW of the mapped Denali fault trace at Kluane Lake, Triassic volcanic rocks of Wrangellia exhibit synkinematic calcite and hematite slip surfaces where reverse-slip indicators are overprinted by strike-slip indicators. Reverse fault mean orientations (x=324/63, n=31) are ~co-planar with strike-slip faults (x=345/70, n=28) and computed kinematic shortening directions are ~co-axial (P_rev=06/049 and P_ss=14/213).

30-40 km south at Telluride and Kimberley Creeks, rocks of Wrangellia are juxtaposed against rocks of the Bear Creek assemblage (newly defined), Dezadeash and Amphitheatre formations. A complex, central principal slip zone (PSZ) is ~10 to 35 m wide and is composed of intercalated protomylonite and cataclasite. Bear Creek ultramafic rocks in the NE PSZ have undergone fault-related dolomite and Cr-rich mica (CRM) alteration to form listwanite. CRMs are present as foliated stringers in protomylonite, veins, and disseminations in the NE PSZ, but also as disseminations in isoclinally folded quartz ± dolomite stringers in metapelite of the SW PSZ. Synkinematic calcite, quartz, carbonaceous clay, and hematite coated slip surfaces are found in the PSZ and enveloping damage zone. At Telluride, dip-slip faults are reverse (x=135/84, n =12) and ~co-planar with strike-slip faults (x=145/85, n=28) with ~co-axial shortening directions (P_rev=03/215 and P_ss=01/004). At Kimberley, reverse (x=325/86, n=4), normal (x=315/81, n=11) and strike-slip (x=323/79, n=33) faults show P_rev=11/049, T_nor=19/017, P_ss=08/187 where strike-slip overprints normal slip, and carbonaceous strike-slip fault rocks are overlain by Eocene-Oligocene Amphitheater conglomerates deposited, and deformed, in an extensional jog (cf. Ridgway et al., 1992).

CRM from two PSZ rock samples yielded ⁴⁰Ar/³⁹Ar ages of 40.2 ± 0.2 and 40.7 ± 0.2 Ma. Although neither CRM kinematic nor paragenetic data are available, these Ar ages indicate that the fault zone was active in the late Eocene, consistent with regionally extensive initiation of strain localization (cf. Roeske et al., 2012) and local sedimentation patterns. Reverse- and normal-slip faults consistently overprinted by strike-slip faults provide a complex, bedrock record of strain compatibility, partitioning, and progressive deformation in these bedrock exposures.