Forearc basins are fundamental components of a subduction system. The Cook Inlet forearc basin is the largest active forearc basin in North America, with dimensions 200 km by 100 km and a depth of 7.6 km. The basin has remained a site of sedimentation and subsidence over the past 160 Ma of convergence and accretion along the southern Alaska margin. The basin is part of a 50-km thick crust overlying a freely slipping plate interface, just downdip from the rupture extent of the Mw 1964 9.2 earthquake. A high-magnetic, low-gravity, continental-scale signal known as the Southern Alaska Magnetic High (SAMH) coincides with Cook Inlet basin. The origin of this signal has been proposed to result from either an ancient accreted magmatic arc within the crust, or a serpentinized zone within the mantle wedge. We assume that the structural body associated with the SAMH has driven forearc basin formation over geological time scales. By characterizing the three-dimensional seismic structure of the basin-crust-slab-mantle system, it should be possible to isolate the origin of the SAMH and, in turn, the origin of a globally significant forearc basin.
Sedimentary basins pose major challenges for seismic imaging. Basin reverberations of seismic waves complicate efforts to image underlying and adjacent portions of the subduction system, including the lower crust, uppermost mantle, and subducting slab. Three components are needed to derive a high-quality seismic structural model of the Cook Inlet region: (1) seismic imaging capabilities to handle both the structural complexities of the three-dimensional model and details within full seismic waveforms; (2) maximal distribution local earthquakes to enhance resolution; and (3) an array of strategically placed broadband seismometers.
We plan a two-year EarthScope FlexArray deployment of 30 seismic stations to target (1) the 50-km thick continental crust containing Cook Inlet basin, and (2) a 400-km long arc-normal transect of the subduction zone across the basin, through Redoubt volcano, and into the backarc region. With a phenomenal rate of seismicity within the crust and subducting slab, the region of Cook Inlet offers a world-class opportunity for seismic imaging of subduction tectonics. By isolating the seismic signals due to the basin, it will be possible to image deeper structure and processes within the lower crust, upper mantle, and subducting Pacific slab. We plan to harness the accuracy of full wavefield simulations within an adjoint-based imaging technique that has proven successful in southern California. A complete characterization of the structural setting will result from detailed investigations of local and teleseismic shear-wave splitting measurements to infer mantle flow patterns, in addition to a synthesis of tectonic stresses inferred from hundreds of M > 3 moment tensors from earthquakes within the crust and slab.
