Transtensional Tectonics of the Minto Flats Fault Zone and Nenana Basin, Central Alaska

Abstract

Abstract Central Alaska is a broad zone of crustal deformation that is produced by collision and flat slab subduction. Within central Alaska, there are large‐scale right‐lateral strike‐slip faults, such as the Denali fault (2002 M w 7.9), as well as smaller‐scale fold‐and‐thrust belts and a set of left‐lateral strike‐slip fault zones, one of which is the Minto Flats fault zone (MFFZ). We use seismological evidence to document a pair of overlapping left‐lateral faults that define the MFFZ. Microseismicity delineates this 180‐km‐long fault zone. Using body waves and surface waves, we perform moment tensor inversions for the 11 best‐recorded earthquakes in the fault zone. Moment tensors reveal consistent left‐lateral faulting throughout the fault zone. A finite‐source model for the 1995 M w 6.0 earthquake is consistent with left‐lateral faulting and provides rupture details for the largest known event in the fault zone. The two main faults are separated by 10 km and overlap by 67 km, forming a releasing stepover geometry within a local transtensional setting. Between the faults is a 90‐km‐long, 12‐km‐wide, and 8‐km‐deep sedimentary basin (the Nenana basin). We interpret the transtensional faulting to be responsible for the development of the basin over the past 6 Ma. The distances of fault overlap and fault separation are key parameters for determining (1) the 3D morphology of the sedimentary basin and (2) the likelihood of earthquake ruptures jumping from one fault to the next. The structure of the Nenana basin is consistent with shear motion accommodated by the identified faults. The 10 km fault separation suggests that ruptures are not likely to span the entire fault zone. Earthquakes as large as M w 7.0–7.5 could occur on the faults. The transtensional fault zone provides an important constraint for understanding the larger‐scale intraplate tectonic setting of central Alaska. Online Material: Figures of waveform fits, depth estimation for moment tensors, variation in seismicity, and fault‐plane identification.