ESCAPE TECTONICS AND PLATE KINEMATICS IN THE SOUTH CENTRAL ALASKAN OROCLINE

Geological field observations across the trace of the Denali fault system (DFS) of southern Alaska reveal incompatibilities between the volumes of material translated into south-central Alaska and the amounts currently there or have feasibly been removed by erosion. Observed strike slip offset across the eastern DFS predicts some ~236 km of coast-parallel shortening in south central Alaska since the early Cenozoic. So much transfer of crustal material requires either extensive underplating, great topographic relief, or extensive removal of upper-crustal material evidenced by widespread exhumation of high-grade metamorphic rocks. Because coast-parallel transport of low-density terrane fragments occurred predominantly along strike slip faults inboard of active subduction, underplating of the terranes is not likely. Extreme topography is limited to the Denali region of the Alaska Range, and because most of south-central Alaska is underlain by sedimentary or low-grade metamorphic rocks, accommodation by crustal thickening (necessarily accompanied by rapid erosional exhumation) also seems unlikely. To accommodate the large-scale coast-parallel transport predicted by plate tectonic relative motions we instead propose that regional scale shortening was effectively annulled by widespread expulsion of terrane fragments towards the west and southwest. Expulsion resulted in the counter-clockwise vertical axis rotations of southwest Alaska that are observed in the paleomagnetic record, and effectively created the arcuate Alaskan orocline. This model is similar in concept to previously published interpretations of modern Anatolian tectonics, and to models explaining the curved nature of margin-parallel strike-slip faults in the Andean forearc.