geosciences Article Basement Structure and Styles of Active Tectonic Deformation in Central Interior Alaska Nilesh C. Dixit 1,* and Catherine Hanks 2 1 Department of Physics, Geology and Engineering Technology, Northern Kentucky University, Nunn Drive, Highland Heights, KY 41099, USA 2 Department of Geosciences, University of Alaska Fairbanks, 900 Yukon Drive, Fairbanks, AK 99775, USA; [email protected] * Correspondence: [email protected] Abstract: Central Interior Alaska is one of the most seismically active regions in North America, exhibiting a high concentration of intraplate earthquakes approximately 700 km away from the southern Alaska subduction zone. Seismological evidence suggests that intraplate seismicity in the region is not uniformly distributed, but concentrated in several discrete seismic zones, including the Nenana basin and the adjacent Tanana basin. Although the location and magnitude of the seismic activity in both basins are well defined by a network of seismic stations in the region, the tectonic controls on these intraplate earthquakes and the heterogeneous nature of Alaska’s continental interior remain poorly understood. We investigated the crustal structure of the Nenana and Tanana basins using available seismic reflection, aeromagnetic and gravity anomaly data, supplemented by geophysical well logs and outcrop data. We developed nine new two-dimensional forward models to delineate internal geometries and the crustal structure of Alaska’s interior. The results of our study demonstrates a strong crustal heterogeneity beneath both basins. The Tanana basin is a relatively Citation: Dixit, N.C.; Hanks, C. shallow (up to 2 km) asymmetrical foreland basin with its southern, deeper side controlled by the Basement Structure and Styles of Active Tectonic Deformation in northern foothills of the Central Alaska Range. Northeast-trending left lateral strike-slip faults within Central Interior Alaska. Geosciences the Tanana basin are interpreted as a zone of clockwise crustal block rotation. The Nenana basin 2021, 11, 127. https://doi.org/ has a fundamentally different geometry. It is a deep (up to 8 km), narrow transtensional pull-apart 10.3390/geosciences11030127 basin that is deforming along the left-lateral Minto Fault. This study identifies two distinct modes of current tectonic deformation in Central Interior Alaska and provides a basis for modeling the Academic Editors: interplay between intraplate stress fields and major structural features that potentially influence the Jesus Martinez-Frias and Lev generation of intraplate earthquakes in the region. V. Eppelbaum Keywords: potential field modeling; interior Alaska; transtensional basin; foreland basin; Received: 15 January 2021 seismic interpretation Accepted: 24 February 2021 Published: 10 March 2021 Publisher’s Note: MDPI stays neutral 1. Introduction with regard to jurisdictional claims in published maps and institutional affil- The observed occurrence of intraplate earthquakes within a rigid continental interior iations. provides direct evidence of localized zones of high strain concentration hundreds of kilometers away from the plate boundary. Although such events account for less than 10 percent of all global earthquakes and release only 5 percent of the global seismic energy, they can inflict heavy damage to human lives and property in the region [1]. From a plate tectonics view, intraplate earthquakes provide important information about the rheology Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. of the lithosphere and tectonic forces and stresses operating within the lithosphere [2,3]. This article is an open access article Investigating relationships among lithospheric strength, tectonic stresses, and intraplate distributed under the terms and seismicity can significantly improve our knowledge of intraplate dynamics, which would conditions of the Creative Commons allow us to accurately perform seismic hazard assessment in an intraplate setting. Attribution (CC BY) license (https:// Central Interior Alaska is one of the most seismically active intraplate regions in creativecommons.org/licenses/by/ the North America. In the past, this region has experienced significant seismic activity, 4.0/). including large-scale earthquakes as high as Mw 7.9. Examples include 2002 Mw 7.9 Denali Geosciences 2021, 11, 127. https://doi.org/10.3390/geosciences11030127 https://www.mdpi.com/journal/geosciences Geosciences 2021, 11, 127 2 of 30 Central Interior Alaska is one of the most seismically active intraplate regions in the Geosciences 2021, 11, 127 North America. In the past, this region has experienced significant seismic activity,2 of 25 in- cluding large-scale earthquakes as high as Mw 7.9. Examples include 2002 Mw 7.9 Denali Fault earthquake; 1967 Mw 5-6 Fairbanks earthquake; 1937 Mw 7.3 Salcha earthquake; 1947Fault Mw earthquake; 7.2 earthquake 1967 Mw in 5-6 the Fairbanks northern earthquake; foothills 1937of the Mw Alaska 7.3 Salcha Range earthquake; [4–6] (Figure 1947 1). BasedMw on 7.2 earthquakeincreasing inseismological the northern foothillsevidence, of intraplate the Alaska Rangeseismicity [4–6 ]in (Figure the Central1). Based Alaska regionon increasing appears seismologicalto not be uniformly evidence, distributed, intraplateseismicity but concentrated in the Central in several Alaska discrete region seis- micappears zones to(Figure not be 1). uniformly The location distributed, and magnitud but concentratede of the in seismic several activity discrete seismicin Central zones Alaska is well(Figure defined1). The by location a network and magnitude of seismic ofstations the seismic in the activity region. in However, Central Alaska the tectonic is well con- defined by a network of seismic stations in the region. However, the tectonic controls on trols on intraplate earthquakes and the heterogeneous nature of the continental litho- intraplate earthquakes and the heterogeneous nature of the continental lithosphere remain spherepoorly remain understood. poorly A understood. detailed knowledge A detailed of the knowledge crustal structure of the beneath crustal Central structure Alaska beneath Centralprovince Alaska is critical province to help is testcritical various to help tectonic test models various explaining tectonic themodels interplay explaining between the in- terplaythe intraplate between stress the fieldsintraplate causing stress earthquakes fields causing and the earthquakes development ofand regional the development structural of regionalfeatures structural in this region. features in this region. FigureFigure 1. Simplified 1. Simplified tectonic tectonic map map of ofcentral central Alaska Alaska showing showing the locationlocation of of the the study study area area relative relative to major to major active active tectonic tectonic featuresfeatures and the and major the major active active faults faults in inth thee region region [7–10]. [7–10]. Red box indicatesindicates the the position position of Figureof Figure 2. The 2. The Minto Minto Flats Flats Seismic Seismic Zone Zone(MFSZ), (MFSZ), the Fairbanks the Fairbanks Seismic Seismic Zone Zone (FSZ), (FSZ), and and the SalchaSeismic Seismic Zone Zone (SSZ). (SSZ The). The horizontal horizontal velocity velocity vectors vectors are are measuredmeasured relative relative to the to thestable stable North North American American craton craton and areare inferredinferred from from the the GPS GPS velocities velociti calculatedes calculated for the for region the region by Fletcherby Fletcher [5]. Focal [5]. Focal mechanism mechanism solutions solutions of of notable notable earthq earthquakesuakes inin interior interior Alaska Alaska indicate indicate N–S N–S directed directed compressive compressive stressesstresses in response in response to the to theplate plate subduction subduction along along the the southe southernrn marginmargin of of Alaska Alaska [4, 6[4,6,11].,11]. The The blue blue solid solid line marksline marks the the flat-slabflat-slab region region showing showing the the subducted subducted ex extenttent of of the the Yakutat microplate microplate [6 ].[6]. In Inthis this study, study, integrated integrated geophysicalgeophysical investigations investigations were were carried carried out toout image to image the the subsurface structures, basement depth, and formation boundaries to construct new crustal subsurface structures, basement depth, and formation boundaries to construct new crus- scale models for the Central Alaska province. The structure of the upper crust was defined tal byscale integrating models gravityfor the andCentral magnetic Alaska field province. data with The newly structure obtained of industrythe upper seismic crust was definedreflection by integrating profiles, geophysical gravity and logs, magnetic and outcrop field data. data The with models newly presented obtained in this industry paper seis- michelp reflection to constrain profiles, the basin geophysical geometries, logs, define and major outcrop boundaries data. The between models the sedimentarypresented in this paperand help underlying to constrain heterogeneous the basin basement geometries units,, define identify major the possible boundaries location between of intrusive the sedi- mentary and underlying heterogeneous basement units, identify the possible location of Geosciences 2021, 11, 127 3 of 25 bodies, and identify possible active geologic structures responsible for the ongoing crustal deformation in the region. Our results show that the Cenozoic sedimentary basins that