FOREARC COOLING DURING TWO DIFFERENT MODES OF FLAT-SLAB SUBDUCTION, INSIGHTS FROM LOW-TEMPERATURE BEDROCK AND DETRITAL THERMOCHRONOLOGY, SOUTH-CENTRAL ALASKA

The forearc of Alaska has been subjected to two unrelated superimposed episodes of flat-slab subduction during Cenozoic time that produced dissimilar magnitudes, patterns, and modes of upper-plate exhumation. The first event from about 61-50 Ma occurred when thermally buoyant oceanic lithosphere at a spreading ridge (Kula-Farallon or Kula-Resurrection ridge) was obliquely subducted beneath North America. A second event beginning in early Miocene time is ongoing, as thick, relatively buoyant crust of an oceanic plateau (Yakutat microplate) subducts beneath and collides with the margin.

We use 49 new and 63 published apatite fission-track and apatite (U-Th)/He ages of exhumed magmatic arc and forearc basement rocks to determine spatial patterns of exhumation over a broad region of the forearc inboard of the accretionary wedge. We also analyzed 27 new paired detrital zircon and apatite fission-track samples from Cenozoic forearc basin sediments and sands from modern rivers draining into the forearc basin to assess temporal changes in exhumation and sediment lag times.

The results indicate that an extensive region encompassing parts of the western and central Alaska Range, Susitna basin area, and the southwestern Talkeetna Mountains cooled primarily between 56 and 39 Ma, synchronously with or shortly following eastward sweeping of the spreading ridge. Late Paleocene-Eocene ages widely persist above the subducted Yakutat crust, including areas of significant topographic relief, indicating that the magnitude of exhumation in the arc-forearc region since its collision has been minor. Miocene and younger cooling is only recorded in narrow, restricted belts generally associated with the highest-relief areas of the Alaska Range near the periphery of the subducted Yakutat microplate. Our results suggest that exhumation in the study area resulting from Yakutat collision of sufficient magnitude to overprint Paleocene-Eocene cooling has been locally concentrated proximal to the Denali fault and small region of the western Alaska Range near the western margin of the southern Alaska block. In contrast, the broad, relatively uniform cooling associated with ridge subduction likely reflects modest widespread exhumation, potentially during elevated heat flow conditions.