THERMOCHRONOLOGICAL CONSTRAINTS ON SMALL-SCALE VARIATIONS IN STRAIN-PARTITIONING ALONG THE DENALI FAULT, OF ALASKA

Topography, basin formation, and rain shadow development have been used as proxies to understand far-field plate tectonic processes in numerous tectonic environments such as the Himalayas. Along major intracontinental strike-slip fault systems such as the Alpine and the San Andreas faults, small scale variations in near-field boundary conditions (e.g. rheology, fault geometry, climatic forcing) play a strong role in asymmetric topographic development and thus basin formation and rain shadow development.

The Denali Fault of Alaska is ~500 km north of the active southern Alaskan margin, where an obliquely convergent subduction zone is complicated by accretion of the Yakutat terrane. Rain shadow development, basin history, and exhumation of the 650 km long Alaska Range along the Denali Fault, have been interpreted in relation to Pacific plate motion changes, micro-plate accretion, and/or block rotation, and have been used to test geodynamic models for flat-slab subduction.

Variations in slip rates, hinterland width, and basin locations point to strain partitioning along the Denali Fault. Preliminary ⁴⁰Ar/³⁹Ar mica, K-feldspar and apatite fission track thermochronologic data from the Alaska Range indicate a pattern of variable, asymmetric, and localized exhumation along and across-strike on a scale of tens of kms. However, in contrast to what we would expect, the greatest exhumation is not in the central Alaska Range, home to the highest topography including Mt. McKinley lying south of the Denali Fault in the concave side of the fault apex. Rather our thermochronologic data in the eastern Alaska Range suggest a region of younger, focused extreme exhumation <10 km north of the fault, in the vicinity of the 2002 M7.9 earthquake.

The constrained pattern of exhumation is inconsistent with a hypothesis that uplifted topography forms homogeneously along the fault and is controlled by the degree of obliquity of plate motion along the margin. Rather it brings to light the importance of near-field (small-scale) features such as variations in fault geometry and rheology being .responsible for the observed heterogeneous exhumation patterns and the need to characterize their influence on topography or basin formation before these, in turn, can be used as proxies to interpret far-field large-scale plate tectonic processes.