Uplift and exhumation of the Russell Fiord and Boundary blocks along the northern Fairweather transform fault, Alaska
Cooling ages of tectonic blocks between the Yakutat microplate and the Fairweather transform boundary fault reveal exhumation due to strike slip faulting and subsequent collision into this tectonic corner. The Yakutat and Boundary faults are splay faults that define tectonic panels with bounding faults that have evidence of both reverse and strike slip motion, and they are parallel to the northern end of the Fairweather fault. Uplift and exhumation simultaneous with strike slip motion have been significant since the late Miocene. The blocks are part of an actively deforming tectonic corner, as indicated by the ~14 1.5 m of coseismic uplift from the M 8.1 Yakutat Bay earthquake of 1899 and 4 m of strike slip motion in the M 7.9 Lituya Bay earthquake in 1958 along the Fairweather fault. New apatite (U Th Sm)/He (AHe) and zircon (U Th)/He (ZHe) data reveal that the Boundary block and the Russell Fiord block have different cooling histories since the Miocene, and thus the Boundary fault that separates them is an important tectonic boundary. Upper Cretaceous to Paleocene flysch of the Russell Fiord block experienced a thermal event at 50 Ma, then a relatively long period of burial until the late Miocene when initial exhumation resulted in ZHe ages between 7 and 3 Ma, and then very rapid exhumation in the last 1 1.5 m.y. Exhumation of the Russell Fiord block was accommodated by reverse faulting along the Yakutat fault and the newly proposed Calahonda fault, which is parallel to the Yakutat fault. The Eocene schist of Nunatak Fiord and 54 53 Ma Mount Stamy and Mount Draper granites in the Boundary block have AHe and ZHe cooling ages that indicate distinct and very rapid cooling between ca. 5 Ma and ca. 2 Ma. Rocks of the Chugach Metamorphic Complex to the northeast of the Fairweather fault and in the fault zone were brought up from 10 12 km at extremely high rates >5 km/m.y. since ca. 3 Ma, which implies a significant component of dip slip motion along the Fairweather fault. The adjacent rocks of the Boundary block were exhumed with similar rates and from similar depths during the early Pliocene, when they may have been located 220 250 km farther south near Baranof Island. The profound and significant exhumation of the three tectonic blocks in the last 5 m.y. has probably been driven by uplift and erosional exhumation due to contraction as rocks collide into this tectonic corner. The documented spatial and temporal pattern of exhumation is in agreement with the southward shift of focused exhumation at the St. Elias syntaxial corner and the southeast propagation of the fold and thrust belt. LITHOSPHERE GSA Data Repository Item 2019033 https:/doi.org/10.1130/L1011.1