Does a Boundary of the Wrangell Block Extend Through Southern Cook Inlet and Shelikof Strait, Alaska?

In southcentral Alaska, the boundaries of two different tectonic blocks extend southwestward from the Denali Fault toward Cook Inlet and Shelikof Strait. We use offshore multichannel seismic reflection data and oil-well stratigraphy to evaluate whether local geologic structures are compatible with boundaries of either tectonic block and with the relative motion expected across the block boundaries. Our main conclusion is that a block boundary does not extend southwestward the entire length of Shelikof Strait, as was proposed for one of the blocks. Furthermore, below southern Cook Inlet, no high-strain extensional structures that might be related to either proposed boundary are evident. Small normal faults below southern Cook Inlet could have been caused by block rotation, but they represent only minor strain. One way to explain the lack of larger structures is that the rotation began recently so that indicative boundary structures have not yet formed. Alternatively, deformation associated with the block boundaries could be distributed through onshore areas.