The Active Yakutat (Kula?) Plate and Its Southcentral Alaska Megathrust and Intraplate Earthquakes

Abstract

Alaska geology and plate tectonics have not been well understood due to an active Yakutat plate, believed to be part of the remains of an ancient Kula plate, not being acknowledged to exist in Alaska. It is positioned throughout most of southcentral Alaska beneath the North American plate and above the NNW subducting Pacific plate. The Kula? plate and its eastern spreading ridge were partially "captured" by the North American plate in the Paleocene. Between 63 Ma and 32 Ma, large volumes of volcanics erupted from its subducted N-S striking spreading ridge through a slab window. The eruptions stopped at 32 Ma, likely due to the Pacific plate flat-slab subducting from the south beneath this spreading ridge. At 28 Ma, magmatism started again to the east; indicating a major shift to the east of this "refusing to die" spreading ridge. The captured Yakutat plate has also been subducting since 63 Ma to the WSW. It started to change to WSW flat-slab subduction at 32 Ma, which stopped all subduction magmatism in W and SW Alaska by 22 Ma. The Yakutat plate subduction has again increased with the impact/joining of the coastal Yakutat terrane from the ESE about 5 Ma, resulting in the Cook Inlet Quaternary volcanism of southcentral Alaska. During the 1964 Alaska earthquake, sudden movements along the southcentral Alaska thrust faults between the Yakutat plate and the Pacific plate occurred. Specifically, the movements consisted of the Pacific plate moving NNW under the buried Yakutat plate and of the coastal Yakutat terrane, which is considered part of the Yakutat plate, thrusting WSW onto the Pacific plate. These were the two main sources of energy release for the E part of this earthquake. Only limited movement between the Yakutat plate and the North American plate occurred during this 1964 earthquake event. Buried paleopeat age dates indicate the thrust boundary between the Yakutat plate and North American plate will move in about 230 years, resulting in a more "continental" type megathrust earthquake for southcentral Alaska. There are, therefore, at least two different types of megathrust earthquakes occurring in southcentral Alaska: the more oceanic 1964 type and the more continental type. In addition, large "active" WSW oriented strike-slip faults are recognized in the Yakutat plate, called slice faults, which represent another earthquake hazard for the region. These slice faults also indicate important oil/gas and mineral resource locations.

Association of the November 30, 2018 M 7.1 Earthquake near Anchorage, Alaska to the Tectonics of Southcentral Alaska

“…The main but not initial megathrust in the eastern part of the great 1964 Alaska M 9.2＋ earthquake was actually the yak thrusting west-southwest onto the Pacific plate (Reeder, 2016;Stauder and Bollinger, 1966;Wyss and Brune, 1967). This deep thrust had a strike azimuth of 344° with a dip of 26° to the NE (Berg, 1965) and is represented by the large thrust fault at the western boundary of yak with the Pacific plate as shown on Fig.…”

“…But back at and beyond the Augustine slice fault (AUG) the PAC anticlinal apex orientation is due south. This directional change was due to the initial west-southwest movement of the Yakutat microplate or terrane (yak) when it collided/locked with the YAK about 5 million years ago (Reeder, 2016). At that time, the yak thrusted west-southwest onto PAC at less rate than the north-northwest movement of the PAC.…”

“…Alaska geology and plate tectonics have not been well understood due to an unacknowledged active Yakutat plate (YAK&yak) that consists of YAK and yak ( Fig. 1; Reeder, 2016). The yak is the Yakutat terrane (also called Yakutat microplate).…”

“…In opposition to the PAC, the YAK part of the Yakutat plate is subducting more slowly to the west-southwest. This subduction increased when the yak attached/joined the YAK in the Prince William Sound region from the southeast about 5 million years ago (Arkle et al, 2013;Cox and Engebretson, 1985;Reeder, 2016).…”

“…1) which has been partially filled by Wrangell volcanic deposits in its southeast region. This moving YAK has pronounced offsets caused by differential movements of large west-southwest elongated blocks named slices that are bound by long strike-slip fault zones, called slice faults (Reeder, 2016). They extend west-southwest through the YAK into the Cook Inlet subduction zone and are associated with its volcanoes (Fig.…”