THE FORMATION OF THE ALASKA RANGE: ALTERNATING ASYMMETRIC TOPOGRAPHY AND EXHUMATION ACROSS THE DENALI FAULT, AND THE ROLE OF INHERITED STRUCTURES, STRAIN PARTITIONING, THRUST FAULTS AND SOUTHERN ALASKAN PLATE BOUNDARY PROCESSES

The Alaska Range formed in the Neogene along the arcuate Denali fault (DF) ~500 km inboard of the southern Alaskan margin. The geometry and location of the DF is controlled largely by pre-existing weakness and structural inheritance between large composite terranes. Strain is transferred to the DF from southern Alaska plate boundary forces that include Yakutat microplate interaction (flat-slab subduction with variable coupling) and variations in relative plate motion. Strain is partitioned into dextral and oblique slip along the DF and as thrust faults N and S of the DF. Partitioning along the DF is reflected by decreasing slip rate to the west and by high-standing topography and greater exhumation south of, and within the McKinley restraining bend (forming the central Alaska Range). There, SW-striking faults including the hypothesized Broad Pass thrust zone likely accommodate the thrust component of transpression. In contrast, in the eastern Alaska Range higher topography and greater exhumation occurs north of the DF on the convex side of the Mount Hayes restraining bend. The enigmatic alternating topographic/exhumation patterns are controlled by the irregular shape of a stronger Yukon-Tanana terrane backstop relative to the location of the active DF that deforms weaker (Mesozoic) suture zone rocks against that backstop. In the eastern Alaska Range, thrusts north of the DF are generally subparallel to the DF and thermochronology data indicate greater exhumation than south of the DF. Thrusts south of the DF (e.g. the active Susitna Glacier thrust fault, SGTF) are likely less significant compared to those in the central Alaska Range to the west. Apatite fission track profiles across the SGTF indicate the fault was not active during the Eocene and early Oligocene, whereas contrasting apatite (U-Th)/He age profiles (~25 Ma footwall, ~15 Ma hanging wall) suggest a mid-Miocene initiation of faulting. This episodic cooling and exhumation is related to thrusting on known and inferred faults that progressively activate due to varying partition of strain along the DF. Faults vary in orientation from sub-parallel to the DF (e.g. McCallum Creek thrust) to those that splay off the DF (e.g. the SW striking SGTF) as the tectonic regime changes from east to west: from strike-slip to transpressional, to more contractional.