THE GEOLOGIC AND PALEOMAGNETIC CONUNDRUM OF LATE CRETACEOUS THROUGH EARLY EOCENE MAJOR STRIKE-SLIP FAULTS IN ALASKA AND NORTHERN CANADA

Far-traveled terranes in Alaska, both with respect to one another and to the North American craton, have come to be widely accepted based on geological and geophysical data. However, the timing and mechanism for transport of terranes remains very controversial. This study summarizes the current state of the geologic and paleomagnetic (Late Cretaceous to early Eocene plate tectonic) conundrum as seen in Alaska.

The geologic and geophysical data identify three major fault systems, the Tintina, Denali, and Border Ranges, and permit very large-scale strike-slip displacements. The contradiction with the paleomagnetic data is that much of this displacement would appear to have occurred prior to 60 Ma. Of the three major fault systems, the Tintina is best constrained, with current estimates of 450 km post-Cretaceous dextral slip. The Denali fault system total slip is poorly known but more than a few 100 km of post-60 Ma dextral slip is difficult to identify. Geophysical data do permit a buried strike-slip fault along the northern margin of Wrangellia, but few constraints exist on the timing of potential offset. The Border Ranges fault system also may have very significant displacement but was not active as a major strike-slip fault system by 52-50 Ma. At geologically reasonable rates, the total post 60 Ma slip would not be more than 500 km. In sum, the post-60 Ma slip may add up to 1000-1200 km of total offset for the outermost part of the accretionary complex. This amount would place the Chugach accretionary complex close to an abundant source of clastic sediments, resolving one geologic discrepancy. Several studies show that the source of this thick clastic wedge is the Coast Plutonic complex, which would require a minimum of 1200 km of offset. Published paleomagnetic studies from the outermost part of the margin, however, would require greater than 2000 km of displacement since 60 Ma. These studies have been ignored in recent literature but represent ideal rocks to test the total maximum slip between the subduction zone and the North American craton, and thus to determine the potential slip on the major fault systems in-between. The authors are currently reexamining the paleomagnetism of the Ghost Rocks Formation on the Kodiak Islands in order to clarify the apparent paleomagnetic and geologic conundrum.