Crustal And Lithospheric Thickness Variation Across Alaska In Advance Of Earthscope Transportable Array

Abstract:

The Alaskan northern Cordillera is situated at the nexus of a major Pacific-North America plate boundary transition, characterized by the geometric complexity of a highly curved strike-slip fault system and ongoing shallow angle subduction. Existing seismic data from stations throughout southern Alaska and northward across the Dalton Highway are analyzed. Preliminary P wave receiver functions are presented to identify lateral variations in crustal thickness. S wave receiver functions reveal velocity discontinuities within the upper mantle, and we identify novel lithospheric structures. In the northern interior below the Brooks Range, 130 km thick lithosphere contrasts with the interior region that has shallow negative conversions (80 km), marking thinner lithosphere in the greater backarc region where heat flow is observed to be high. In the southeast Coastal Ranges, we interpret a thinner (80-90 km) North American lithosphere above a deeper interface that represents the base of the colliding Yakutat microplate. These imaging results provide context for the distribution of strain throughout the Alaskan Orocline. The presence of thick lithosphere below the Brooks Range may indicate strong lithosphere that deflects strain into central and southwestern Alaska. Thin lithosphere in east Alaska and adjacent Yukon Territory coincides with the occurrence of inboard crustal seismicity, and may be indicative of transmitted compression caused by the collision of the Yakutat microplate. The spatial distribution of the subducted portion of the Yakutat is found to lie below the Wrangell Volcanic Field and St. Elias Mountains. We establish new tectonic elements within a complex lithospheric system, and produce hypotheses that will be suitable for testing with upcoming seismic installations.