PLIOCENE UPLIFT OF THE CENTRAL ALEUTIAN ARC: EVIDENCE FROM THE ~35-31 MA HIDDEN BAY AND ~ 14 MA KAGALASKA PLUTONS ON ADAK ISLAND

New U-Pb zircon, K/Ar, Ar/Ar, fission track and (U-Th)/He ages from the ~ 35-31 Ma Hidden Bay pluton on southern Adak Island and the ~14 Ma Kagalaska pluton just south of the modern Aleutian Arc on Adak provide new constraints on the Oligocene to Pliocene uplift history of the Central Aleutian arc. In detail, a Hidden Bay granodiorite (BW8-1) with a U-Pb zircon age of 32.8±0.84 Ma has an apatite fission track age of 25.8±5.6 Ma and an (U-Th)/He apatite age of 3.17±0.33 Ma and a Kagalaska leucogranodiorite (KAG8-32) with a U-Pb zircon age of 13.65±0.37 Ma has an (U-Th)/He apatite age of 4.17±0.28 Ma. Highly defect apatite grains inhibited resolving a fission track age for the Kagalaska pluton. Given a crystallization depth of ~3-4 Km (Kay et al., 2019), the 35-31 Ma Hidden Bay pluton in the south was at ~3-2 km at ~26 Ma and reached the surface at 3.2 Ma approximately some 1 million years after the ~14 Ma Kagalaska pluton to the north, which was at ~2-3 km at 12.4 Ma. This uplift of Adak Island predates the Pleistocene-Recent arc and is in accord with the suggested upper Pliocene depositional age for the basal detritus of the marine Ua member in the Central Aleutian Upper Series (Harbert et al, 1986; Vallier et al., 1994). A ~ 4 Ma surface uplift age for the Kagalaska pluton further corresponds with an abrupt shift in Bering Sea opal productivity supporting a role for early Pliocene orographic uplift in the abrupt decline of Bering Sea diatom productivity at 4.1 Ma (Stroynowski et al. 2015). A 5-3 Ma timeframe also occurs at a time of high rates of uplift in the region of the Fairweather fault in southeastern Alaska (Schartman et al., 2018). On a larger scale, Austermann et al. (2011) have argued for an abrupt 10-13° clockwise rotation of the subducting Pacific plate at ~ 6-5 Ma that could contribute to surface ascent of the Adak plutons. More recently, DeMets and Merkouriev (2016) have suggested a more gradual rotation of the Pacific plate since 19.7 Ma resulting in an increasing rate of slab-area consumption under the Aleutians since ~ 16 Ma. A combination of increased erosion from the arc and sediment input into the trench related to uplift and increased glacial activity in south central and southeast Alaska have changed the sediment input into the Aleutian trench since the Pliocene.