EVIDENCE FOR AND AGAINST MID-CRUSTAL DETACHMENT HORIZONS BASED ON CA. 25 MYR OF HETEROGENEOUS TRANSPRESSION IN THE DENALI FAULT SYSTEM

The existence of a crustal detachment horizon has been invoked to link distributed deformation throughout the northern Cordillera to post-30 Ma flat slab subduction at the southern Alaska plate margin. In southern Alaska, transpressional sections of the Denali fault system have been exhuming high grade metamorphic rocks since ca. 30 Ma. The fabric ages and exhumation depth of these rocks provide a way to test for the existence of the hypothesized detachment horizon.

We present a synthesis of our recent work using ⁴⁰Ar/³⁹Ar and (U-Th)/He thermochronometry and rock fabric analysis to decipher the Oligocene-Quaternary evolution of shortening structures on the south side of the Denali fault in the eastern Alaska Range. Our data show that quartz-mica schists exhumed by slip on reactivated terrane-bounding structures have cooled from >400ºC since ca. 30 Ma. In contrast, rocks exhumed by thrusts within a single terrane have cooled from ~130ºC since 30 Ma.

Integrating our data with published cooling ages, regional seismicity, and balanced cross sections yields contradictory predictions regarding the mechanics of strain transfer in the crust and addresses the proposed existence of a plate-scale crustal detachment horizon in the northern Cordillera. The depth of exhumation associated with reactivated terrane-bounding faults suggests that exhumation of mid-crustal rocks is controlled by horizontal rheological contrasts within the crust (i.e., the terrane boundaries). In contrast, earthquake hypocenters and balanced cross-sections of the within-terrane thrust systems north and south of the Denali fault suggest the splay thrust systems root into the Denali fault at the base of the seismogenic zone. The non-uniform Quaternary shortening rates across these thrust systems predict effective ‘decapitation’ of the Denali fault via slip along a sub-horizontal fault directly below the seismogenic zone. However the present data suggest that detachment horizons in the middle crust may be relatively ephemeral features, spatially restricted to regions actively undergoing heterogeneous strain. This result suggests that if long-lived plate-scale detachment horizons are kinematically required to transfer strain hundreds of kilometers inboard of the plate boundary in the northern Cordillera, they are not present in the crust.