MESOZOIC-CENOZOIC MAGMATIC HISTORY OF THE ALASKA RANGE SUTURE ZONE

We apply zircon U-Pb geochronology (modern detrital and bedrock) and whole rock ⁴⁰Ar/³⁹Ar geochronology to mafic dikes and volcanic rocks, over a ~200 km sampling transect, to better capture the temporal magmatic history of the Alaska Range Suture Zone. We apply geochemistry on zircons and mafic whole rocks to gain additional insights on the petrogenesis of these plutonic and volcanic rocks.

The Alaska Range Suture Zone has experienced a protracted and complex Mesozoic-Cenozoic magmatic history, with discrete magmatic pulses and lulls. Between ~100-90 Ma a suite of plutons was emplaced in the suture zone during accretion of the Wrangellia composite terrane against North America. This pulse of magmatism was followed by a relative hiatus until ~70 Ma, when a ~15 MY period of wide spread arc magmatism occurred as the proto-Pacific plate underwent oblique subduction along its northeast margin. Additionally, there was a period of magmatism between ~62-50 Ma with intraplate characteristics, atypical of arc processes, possibly a result of post-collisional delamination beneath the suture zone and likely transtentional related magmatism along the Denali Fault during this time period.

From ~55 Ma to ~45 Ma there was another lull in magmatism that is associated with a Paleocene-Eocene ridge subduction event. ~40 Ma to ~30 Ma was a period of high flux magmatism with arc characteristics along the Alaska Range Suture zone as a young-hot Pacific plate underwent flat-slab subduction, possibly in part driven by the hydrostatic affects of the over thickened Alaska Range Suture Zone. Magmatism along the Alaska Range arc waned between ~30 Ma and ~25 Ma, limited primarily to mafic dike-emplacement along the north side of the Denali Fault zone, likely reflecting a transitional period between Pacific plate subduction and flat-slab subduction of the Yakutat microplate. There is sparse arc volcanic activity in the region at ~7 Ma, ~1 Ma, and during the Holocene. Shutting down of the central Alaska Range arc is coincident with the ~26 Ma initiation of the Wrangell arc along the edge of the Yakutat flat-slab. The lack of significant magmatism along the Alaska Range arc during the Neogene is likely reflecting both the affects of the flat-slab subduction of the Yakutat microplate and the resultant transpressive nature of the central Denali Fault system.