Imaging the transition from Aleutian subduction to Yakutat collision in central Alaska, with local earthquakes and active source data

Abstract

[1] In southern and central Alaska the subduction and active volcanism of the Aleutian subduction zone give way to a broad plate boundary zone with mountain building and strike-slip faulting, where the Yakutat terrane joins the subducting Pacific plate. The interplay of these tectonic elements can be best understood by considering the entire region in three dimensions. We image three-dimensional seismic velocity using abundant local earthquakes, supplemented by active source data. Crustal low-velocity correlates with basins. The Denali fault zone is a dominant feature with a change in crustal thickness across the fault. A relatively high-velocity subducted slab and a low-velocity mantle wedge are observed, and high V p /V s beneath the active volcanic systems, which indicates focusing of partial melt. North of Cook Inlet, the subducted Yakutat slab is characterized by a thick low-velocity, high-V p /V s crust. High-velocity material above the Yakutat slab may represent a residual older slab, which inhibits vertical flow of Yakutat subduction fluids. Alternate lateral flow allows Yakutat subduction fluids to contribute to Cook Inlet volcanism and the Wrangell volcanic field. The apparent northeast edge of the subducted Yakutat slab is southwest of the Wrangell volcanics, which have adakitic composition consistent with melting of this Yakutat slab edge. In the mantle, the Yakutat slab is subducting with the Pacific plate, while at shallower depths the Yakutat slab overthrusts the shallow Pacific plate along the Transition fault. This region of crustal doubling within the shallow slab is associated with extremely strong plate coupling and the primary asperity of the M w 9.2 great 1964 earthquake.

Megathrust splay faults at the focus of the Prince William Sound asperity, Alaska

“…Although we would expect to see a shallowing of the megathrust surface toward the trench, we attribute the increase in megathrust depth as due to the transition from Yakutat/ Pacific subduction beneath PWS to only Pacific plate subduction southwest of Montague Island. This gentle southwest dip of the top of the seismogenic zone has been estimated from relatively low-resolution tomography data [Eberhart-Phillips et al, 2006], but here we show with much higher resolution that the megathrust is indeed gradual and no large-scale midcrustal step is present at the Yakutat terrane boundary. This gradual transition contrasts with the Yakutat/Pacific plate boundary along the Transition fault beneath the eastern Gulf of Alaska, where an abrupt change in crustal thickness is observed [Christeson et al, 2010] [31] Between positions 30 km and 90 km on the TACT profile, we observe additional reflectors below the megathrust that form a lens-shaped zone of reflective rocks ( Figure 5).…”

“…Based on the near-surface tectonic expression, Wang and Hu [2006] and Kimura et al [2008] would characterize this region as the transition zone of subduction between the inner and outer wedge zones that accommodates the bulk of subduction zone shortening. Although the transition zone is typically defined between the outer arc high and continental slope, we suggest that the shallow subduction angle from the buoyant Yakutat slab above the Pacific plate changes the subduction and splay fault geometry [e.g., Bruns, 1983;Brocher et al, 1994;Eberhart-Phillips et al, 2006;Fuis et al, 2008], pushes this transition zone closer to mainland Alaska for the PWS asperity, and provides the buoyancy to form the barrier islands of PWS. Assuming Montague Island defines a region of coseismic strengthening at the outer arc high and the trailing edge of the Yakutat terrane provides midcrustal heterogeneities and/or duplexing for splays to diverge from the megathrust, we outline an area with the maximum plate coupling where the faults will coseismically rupture during most great earthquakes (Figure 6).…”

“…This depth also correlates with a boundary showing a large seismic velocity contrast [e.g., Brocher et al, 1994;Oleskovich, et al, 1999;Doser and Veilleux, 2009;Fuis et al, 2008]. The earthquake initiated at the zone between the downgoing Yakutat and Pacific plates, but rupture propagated along splay faults through the subducted Yakutat terrane and overlying accretionary complex [Plafker, 1969;Brocher et al, 1994;Eberhart-Phillips et al, 2006;Fuis et al, 2008]. The earthquake shifted portions of PWS southeast as much as 21 m and lifted portions of the region more than 12 m (Figure 2) [Plafker, 1969].…”

“…West of Montague Island, the subducting Yakutat slab is absent and the Pacific Plate subducts directly beneath the North American plate [e.g., Brocher et al, 1994;EberhartPhillips et al, 2006]. The PWS asperity, defined as a region of high moment release [e.g., Lay et al, 1982;Scholz and Campos, 2012], was centered beneath the southwest end of Montague Island near a prominent magnetic high that defines the western boundary of the subducted Yakutat terrane (Figure 1) [Bruns, 1983;Griscom and Sauer, 1990;Brocher et al;1994;Johnson et al, 1996;Zweck et al, 2002;Eberhart-Phillips et al, 2006].…”

“…The Yakutat microplate is relatively buoyant, which results in a subduction angle of approximately 3°beneath PWS compared to the steeper 8°dip along the Kodiak segment [e.g., Brocher et al, 1994;Eberhart-Phillips et al, 2006;Doser and Veilleux, 2009]. The maximum slip from the 1964 earthquake was largely coincident with the southwestern edge of the subducted Yakutat terrane, which appears to be largely coupled to the underlying Pacific plate [Zweck et al, 2002;Doser et al, 2004;Eberhart-Phillips et al, 2006;Ichinose et al, 2007]. Geodetic measurements in the PWS area show movement at the Pacific-North America plate rate, which indicates a completely locked asperity [Zweck et al, 2002] with repeat times for large megathrust earthquakes of 330-900 years (summary in Carver and Plafker [2008]).…”