The subduction of the Yakutat oceanic plateau and its crustal properties

Abstract:

The Yakutat terrane, a thick oceanic plateau is subducting beneath the North America plate and plays an important role in the tectonic evolution of the southern Alaska through collision in the east and flat subduction zone to the north and west. In this study, we invert receiver functions of 39 permanent broadband stations above the Yakutat microplate using the Neighborhood Algorithm, to investigate the crustal properties, such as thickness, Vp/Vs ratios, layer dipping and anisotropic properties. Our goal is to trace the subduction of the Yakutat oceanic plateau. These 39 stations can be divided into 3 regions: a collision area in the east, the shallow subduction area to the north and deeper subduction to the west. In the collisional area, high topography on the surface suggests compressive interactions between the Yakutat terrane and North America plate. The crustal thickness here varies from 27.3 km to 50.3 km with Vp/Vs ratio from 1.66 to 2.10. The crustal thickness decreases to 19.3 km in the shallow subduction area where there are thick low velocity sedimentary layers on top of the Yakutat basement. Except for 3 stations with Moho depth shallower than 30 km and 1 station exceeding 40 km, most stations here are within 30 to 40 km with Vp/Vs ratio from 1.70 to 1.84. In the deeper subduction area, the crustal thickness shows a westward thickening from 37.6 to 51.1 km with Vp/Vs varying from 1.71 to 2.13. In this area, a thick low-velocity layer can be detected beneath several stations, whose thickness varies from 12.9 to 21.6 km and is consistent with the thickness of the subducted Yakutat oceanic plateau. A very thin low velocity layer (1.3 to 4.5 km thick) is also detected beneath the coastal stations and may represent the subducted sedimentary layer. The dip angle and directions of the layer above Moho varies dramatically, indicating complex accretion process during the subduction of Yakutat terrane. Anisotropy properties show less dominant influence than layer dipping in our inversion result though in some stations, anisotropy plays important roles in velocities. This study provides constraints on geometry and constitution of the subducting Yakutat oceanic plateau, linking the offshore Yakutat terrane to the inland flat subducting slab.