VARIABLE STRAIN PARTITIONING WITH DEPTH ALONG THE DEXTRAL DENALI FAULT IN THE EASTERN ALASKA RANGE
Our structural and low-T thermochronologic investigations in the region of the 2002 ruptures indicate that oblique convergence and strain partitioning in the brittle crust has been an ongoing process since ~24 Ma, but unlike the 2002 event, maximum exhumation has been accommodated primarily on the north side of the DF, resulting locally in exhumation of rocks from >~15 km. Low T-thermochronology puts an upper limit on possible depth of exhumation south of the DF at ~5 km during the same time period.
The structural response to the convergent component changes with temperature and crustal depth. Orthogneiss and country rock deformed at amphibolite facies have a consistent strong, steeply-N dipping foliation subparallel to the modern strike-slip DF and contain mineral lineations with ~20-45° WNW rakes. We interpret this as indicating exhumation occurred by transpression in the ductile regime, with possible minor strain partitioning. The amount of strain partitioning increases in rocks deformed at lower T, approaching the brittle-ductile transition, with the pitch of lineations being subhorizontal on steeply N-dipping foliation planes and close to down-dip on moderately north-dipping foliation planes. Brittle deformation has been occurring since at least the mid-Miocene on the SGTF and continues to occur along other NE-striking thrust faults N and S of the DF, as well as normal faulting along NNW-striking faults north of the DF.
The overall width of the region accommodating convergence has varied in time and space, but the strike-slip component of the fault system has apparently stayed locked in a narrow (<1 km wide) fault zone for over 20 m.y. That, along with the fact that the rocks north of the fault are not moving laterally with respect to the fault curve, may help to explain why a long-lived record of exhumation is preserved immediately north of the DF, despite it being a dominantly dextral slip fault.