NEOGENE TRANSPRESSIONAL BASIN DEVELOPMENT ALONG THE CENTRAL DENALI FAULT SYSTEM: JARVIS CREEK AND MCCALLUM BASINS, ALASKA

The central Denali fault system (CDF) is a westward curving strike-slip fault where local shear displacement has been accommodated by shortening normal to the fault. These transpressional processes are recorded in thrust belts and related foreland basins located both north and south of the CDF. Analysis of new stratigraphic and provenance data, and ⁴⁰Ar/ ³⁹Ar geochronology of tephras from two Neogene transpressional foreland basins separated by the CDF in the Delta River area provides new insight into the Neogene displacement history of the Denali Fault system.

The Jarvis Creek basin is located 30 km north of the fault and contains 400 m of mainly Upper Miocene fluvial sandstone and coal. Paleoflow indicators record north-northwest sediment transport directions. These north-dipping strata show minimum post-Late Miocene deformation with maximum dips of only 21°. New detrital zircon geochronology (n = 498 grains) from sandstone indicates the sources of these sediments have dominant ages of 69, 98, 371, and 1839 Ma. Bulk detrital zircon age distributions are represented by Cenozoic <1%, Mesozoic 64%, Paleozoic 26%, and Precambrian 10%.

The McCallum basin located 15 km south of the CDF contains lacustrine, braided stream, and alluvial fan strata with a minimum thickness of 305 m. Paleocurrent indicators record southwest sediment transport. The maximum depositional ages of the strata are ~ 6-4 Ma based on new tephra ages and detrital zircon geochronology from sandstone. These strata are folded and locally have vertical and overturned dips indicative of Pliocene to Recent deformation. New detrital zircon geochronology (n = 1321 grains) from sandstone indicates the sources of these sediments have dominant ages of 5, 25, 50, 91, and 195 Ma. Bulk detrital zircon age distributions are represented by Cenozoic 28%, Mesozoic 58%, Paleozoic 7%, and Precambrian 7%.

Comparing the amount of deformation between the Jarvis Creek and McCallum basins suggests that Pliocene-Pleistocene transpressional deformation on the CDF is accommodated mainly on the south side of the fault. Differences in the stratigraphy, provenance, and timing of deformation between both basins suggest that they may have developed distal to each other and that their current proximity is related to post-depositional fault juxtaposition.