T230 Tectonic Development of the Northern North American Cordillera

Wednesday, 12 October 2011
8:00 AM - 12:00 PM
Room 200DE

TOPOGRAPHIC AND EXHUMATION ASYMMETRY OF THE ALASKA RANGE RELATIVE TO THE DENALI FAULT: A REFLECTION OF LITHOSPHERIC HETEROGENEITY

Paper number: T230-4
Program: Technical Sessions
Time: 8:50 AM – 9:05 AM

Authors

Abstract

Alaska is composed largely of accreted tectonostratigraphic terranes, resulting in lithospheric and rheological heterogeneity. The Denali fault (DF) system, a major right-lateral transpressional fault system, crosscuts south-central Alaska. The Alaska Range, formed along the DF–a fundamental zone of weakness accommodating deformation, sits partially astride a Mesozoic collision zone, with the Yukon-Tanana composite terrane (YCT) to the north and the Wrangellia composite terrane (WCT) to the south. The YCT (~27 km thick crust) is comprised of ductiley deformed tectonically dismembered Proterozoic to Paleozoic metamorphic rocks and plutonic remnants of a volcanic arc, representing a former continental margin. The WCT (~30 km thick crust) represents an oceanic plateau plus arc rocks. Crust between the YCT and WCT is thicker (~37 km), less mafic and more transitional, comprising a complex mix of Mesozoic rocks overlain by Mesozoic Kahiltna Basin and Paleocene-Eocene Cantwell strata. Magnetic maps delineate well the different tectonostratigraphic units, and also reflect local heterogeneity.

An outstanding issue in Alaskan tectonics is, with respect to the DF, what controls the asymmetry of Alaska Range topography. High topography (and greatest exhumation) is north of the DF in the eastern Alaska Range (EAR), yet south of the DF in the central Alaska Range (CAR). The trace of the DF defines a small circle, skirting the YCT’s southern irregular margin. The Hines Creek fault marks a tectonostratigraphic boundary, the YCT’s southern limit and northern limit of most of the ranges high topography. We propose that asymmetric uplift of, and deformation within, the Alaska Range is controlled in the first order by weaker lithosphere south of the YCT (representing a strong backstop) relative to the DF. Thus, in the EAR where the DF is south of the YCT cutting across an embayment of weaker lithosphere, the greatest deformation (uplift) is north of a concave bend in the DF, between it and the Hines Creek fault. In the CAR, deformation (uplift to form Denali, 6194m) is accommodated south of a concave bend in the DF, as the DF there cuts along the southern margin of the apparently stronger YCT. Deformation (uplift/exhumation) asymmetry relative to the DF thus forms between stronger and weaker lithosphere, but as a result of slip along the DF.