INTERPLAY BETWEEN ARC PLUTONISM AND STRAIN PARTITIONING ALONG THE DENALI FAULT, ALASKA

The ~2000 km long lithospheric-scale dextral Denali fault varies in azimuth > 50° across central AK and is a highly partitioned system. Convergence is accommodated through thrust fault splays on the south, whereas shortening and dextral slip north of the Denali fault is accommodated in the northern Alaska Range fold and thrust belt and bookshelf faulting resulting in clockwise rotation of crustal blocks. Eocene plutonic rocks, which straddle the arcuate Denali fault for > 400 km, are spectacularly exposed and knowing the structural context makes them excellent targets for evaluating tectono-magmatic interactions along a strike slip fault during arc magma transport and emplacement. Herein, we integrate detailed mapping and major and trace element chemistry, with high-precision CA-ID-TIMS U-Pb zircon geochronology from two plutons on opposing sides of the Denali fault to evaluate the role that kinematic setting plays during the growth of individual plutonic systems.

The McGonagall Pluton preserves asymmetric textural zonation and is divided into a series of blocks delineated by southwest striking subvertical to steeply dipping syn-magmatic sinistral shear/fault zones. Porphyritic granodiorite grades to equigranular granodiorite from north to south, with the equigranular map unit widening in map view from east to west within each structural block. Numerous E-W oriented dikes in both units indicate that pluton growth occurred via magmatic crack-seal. Crystallization ages decrease toward the southwest corner of each structural block, compatible with pluton growth during clockwise rotation of individual blocks with an aerial addition rate of <50 m²/yr. In contrast, the Foraker pluton is a texturally homogeneous, symmetrically zoned, and was emplaced into the hanging wall of a reverse fault splay off of the Denali fault. U-Pb analysis indicates that crystallization decreases in age toward the center of the intrusion with an areal addition rate of > 100 m²/yr. Evidence for discrete magma injections, internal contacts, or flow foliations is lacking. These data demonstrate how local kinematic settings and strain partitioning can exert control over mechanisms of magma transport and storage, and the role areal addition rates have on preserving primary internal intrusive relationships.