EXHUMATION WEST OF THE EASTERN DENALI FAULT–TOTSCHUNDA FAULT “FREEWAY JUNCTION” IN SOUTH-CENTRAL ALASKA: TEMPORAL AND SPATIAL PATTERNS ADDRESSING THE RELATIVE IMPORTANCE OF TERRANE RHEOLOGY VS FAULT GEOMETRY

In south-central Alaska, strain along active transpressive faults is largely a response to far-field convergent processes along the Alaskan southern margin (flat slab subduction, microplate collision, relative plate-motion change). The intersection of the Totschunda fault (TF) with the Denali fault (DF) system represents an active dextral “freeway junction”. There, the DF separates the Wrangellia composite terrane (oceanic plateau-arc affinity) to the south from the Yukon composite terrane (continental affinity); two terranes of contrasting lithospheric rheology. We present low-temperature thermochronology (LTT) data from the McCallum-Slate Creek fault–DF region west of the freeway junction. North of the DF, topography is rugged with significant relief (Mt. Kimball, 3139 m) and sample coverage is mostly proximal to the DF. Inverse thermal models indicate episodic cooling (inferred exhumation) in the Eocene, mid- and late-Miocene, and Pliocene (~2.5 Ma). South of the DF topography is less rugged (Peak .8100, 2469 m) and sample coverage more extensive. There, cooling and exhumation patterns are associated with a reactivated fault system, reflecting thrusting and associated wedge-top basin development. Modeling constrains episodic cooling in the Late Cretaceous (~70-60 Ma), Late Miocene (~6 Ma), and Pliocene (~2.5 Ma), with subsidiary events in the Oligocene (~25 Ma) and middle Miocene (~15-10 Ma). Immediately south of the DF, AFT ages decrease stepwise from ~90-60 Ma east of the Chistochina Gl. to <~10 Ma west of there, a result of fault geometry once samples are restored eastwards to Late Miocene positions (~12 mm/yr Holocene slip rate). The older eastern group restore near the DF-TF intersection (paleo-releasing bend?), whereas the western group (<~10 Ma) lie west of the DF-TF intersection along a transpressive uplift fault segment where Miocene exhumation was greater. We infer the ~2.5 Ma event is related to alignment of the TF with the Fairweather transform to the south and the initiation of the Connect fault, as the TF was translated along the DF system. Changes in plate boundary conditions and the large-scale evolution of the DF system control the timing of exhumation events with focused exhumation along the DF controlled by both terrane rheology and local fault geometry, although at different scales.