Research Paper THEMED ISSUE: Geologic Evolution of the Alaska Range and Environs GEOSPHERE Vein formation during progressive Paleogene faulting and folding within the lower Cook Inlet basin, Alaska GEOSPHERE; v. 14, no. 1 J. Rosenthal1,2, P. Betka1,3, E. Nadin2, R. Gillis1, and J. Benowitz4 1Alaska Division of Geological and Geophysical Surveys, 3354 College Road, Fairbanks, Alaska 99709, USA http://doi.org/10.1130/GES01435.1 2Department of Geosciences, University of Alaska Fairbanks, 900 Yukon Drive, Fairbanks, Alaska 99775, USA 3Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9w, Palisades, New York 10964, USA 13 figures; 1 table; 1 supplemental file 4Geophysical Institute, University of Alaska Fairbanks, 903 N Koyukuk Drive, Fairbanks, Alaska 99709, USA CORRESPONDENCE: [email protected] ABSTRACT 1979; Haeussler et al., 2000; Trop et al., 2005). Less well studied at these sites is CITATION: Rosenthal, J., Betka, P., Nadin, E., Gil‑ the evolution of fracture systems that are associated with development of local lis, R., and Benowitz, J., 2018, Vein formation during progressive Paleogene faulting and folding within the Regionally persistent vein sets cut Early Jurassic through late Paleogene(?) and regional structures and that record changes in the regional state of stress lower Cook Inlet basin, Alaska: Geosphere, v. 14, no. 1, strata throughout a study area >2000 km2 in the lower Cook Inlet forearc ba- with time. Note that fractures in this study are either joints (open fractures) or p. 23–49, http://doi.org/10.1130/GES01435.1. sin of Alaska. Using field, aerial, and GIS–based studies, we document vein veins (filled fractures). Henceforth we distinguish fractures as either joints or orientations, group them into four dominant sets, and present relative timing veins, as the cohesion of a crack controls the mechanical behavior and perme- Science Editor: Raymond M. Russo observations to demonstrate their development during regional faulting and ability of the bedrock in which it occurs. Guest Associate Editor: James V. Jones folding of Cook Inlet basin strata. All veins were restored about regional folds Southern Alaska comprises a series of accreted terranes (Fig. 1). In the Received 28 September 2016 by first removing the bedding dip, and then rotating the bedding strike into Cook Inlet forearc basin (Figs. 1 and 2) large-scale structures deforming the Revision received 15 June 2017 parallelism with the regional structural trend (038°). The most dominant vein basin principally occur offshore, and few publicly available subsurface data Accepted 25 September 2017 set strikes ~310°, orthogonal to the regional structural trend, and is present in sets exist for their comprehensive structural analysis. Exposed for at least Published online 22 November 2017 all strata in the field area. The other sets strike 210°, 360°, and 260°. Fold-test 100 km in the Iniskin-Tuxedni region of the Cook Inlet is a systematic regional results show that variations in the vein set orientations throughout the study network of fractures that are well expressed approximately at the magmatic area are correlated with the changes in bedding attitudes that define regional arc-forearc basin boundary that is partly defined by the Bruin Bay fault sys- folds, indicating that the veins formed progressively with the folds. We doc- tem (BBFS; Figs. 1–3). The fractures cut forearc strata of Jurassic through late ument abutting and crosscutting relations between sets, and present a new Paleogene(?) (possibly into Neogene) age, and therefore may record multiple ca. 52 Ma 40Ar/39Ar age of a dike that parallels the dominant set (310°) and is episodes of forearc deformation driven by several documented tectonic events crosscut by others. Based on field relations, relative timing constraints, and along the margin. the fold-test results, we suggest a sequence of vein development and its re- Fractures have long been known to be associated with folds and faults, their lationship to fold growth within the Bruin Bay fault system during Paleogene orientations recording the sequential strain history of a region during evolv- deformation along the southern margin of Alaska. Our results may serve as ing deformation (e.g., Price, 1966; Reches, 1976; Bergbauer and Pollard, 2004; a case study for linking vein development to tectonic events in other ancient Ahmadhadi et al., 2008; Pastor-Galán et al., 2011; Lacombe et al., 2011; Weil and modern forearc basins. and Yonkee, 2012). Such studies tend to focus on inferring sequential stress OLD G regimes for individual fracture sets based on their orientations and relative ages determined through stratigraphic controls and crosscutting and/or abut- INTRODUCTION ting relationships (e.g., Engelder and Geiser, 1980; Hancock, 1985; Engelder, 1987). Pervasive mesoscopic structures (cleavage, joint sets, paleomagnetic OPEN ACCESS Sediment filling forearc basins provides a record of subsidence and exhu- fabrics) have been quantitatively related to progressive stages of deformation, mation driven by convergent margin tectonics. The tectonic processes during namely in the development of folds and faults (Fischer and Wilkerson, 2000; forearc basin evolution can be interpreted by examining the style and chronol- Yonkee and Weil, 2010a, 2010b; Weil and Yonkee, 2012; Li et al., 2013). Thus, ogy of preserved structures that deform such basins. This is particularly true for mesostructures (~0.1–10 m) can now be seen as evolving alongside regional long-lived continental subduction margins, which can undergo deformational structures during progressive tectonic activity. events such as accretion, subduction erosion, and/or subduction of spreading Within the Cook Inlet basin (CIB) study area, Detterman and Hartsock This paper is published under the terms of the ridges. Structural studies in forearc settings often focus on major fault and fold (1966) reported two regional joint sets, one striking 305° and the other striking CC‑BY‑NC license. systems that deform the basins and their margins (e.g., Dickinson and Seely, 225°, roughly orthogonal to each other, and speculated that the joints may be © 2017 The Authors GEOSPHERE | Volume 14 | Number 1 Rosenthal et al. | Fracturing in the lower Cook Inlet basin, southern Alaska Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/14/1/23/4048404/23.pdf 23 by guest on 03 October 2021 Research Paper UPLIFTED MESOZOIC TO SOUTH-CENTRAL CENOZOIC BASINS ALASKA TERRANES AXES OF JURASSIC AND QUATERNARY ARCS KB Kahiltna basin YM Yakutat microplate Quaternary to Recent SUBSIDING MESOZOIC TO AA Aleutian Arc QUATERNARY BASINS CPWCT Chugach-Prince Cook Inlet basin Late Cretaceous to Neo- William composite terrane KNA CB Copper River basin gene Alaska Range arcs SB Susitna basin Early to Late Jurassic Wrangellia composite TA Figure 1. Regional maps of (A) Alaska with 170°W Talkeetna Arc 150°W terrane inset (shaded rectangle) showing location A Approx. subducted Yakutat of (B) tectonic setting of south-central Alaska microplate edge Alaska, including terranes named in the water 250 km (Eberhart-Philips et al., 2006) text. The study area is outlined by the S.A. nada U. white dashed polygon with black back- Ca ground. The map includes elements of 60°N Area of Figure 1B the Cook Inlet basin and related(?) Ce- TA Yukon composite 62° nozoic depocenters, approximate terrane Gulf of terrane boundaries, and the schematic locations Alaska KNA of Late Jurassic to modern magmatic and volcanic arc axes (thick gray dashed lines). KB SBS CMF Extent of subducted Yakutat microplate is CB Wrangellia composite terrane approximated from Eberhart-Phillips et al. (2006) as thin gray dashed outline. SB— n Denali faul TananaTa basinnana basin late Paleocene to modern Susitna basin; CB— Eocene(?) to modern Copper basin; t basin KB—Late Jurassic–Early Cretaceous Ka- le BR hiltna basin; AA—Quaternary–Recent I F t k CPWCT Aleutian Arc axis; KTA—Cretaceous– BBF Neogene arc axis; TA—Early–Late Juras- TA CookCo InletInl basi sic arc axis; BBF—Bruin Bay fault system; CMF— Castle Mountain fault; BRF—Border Ranges fault system; CPWCT—Chugach– 60° Prince William composite terrane; YM— AA Yakutat microplate. Modified from Trop et GulfGulf of AlaskaAlaska YM al. (2003). rruusstt h Fig.Fig. 2 PaPacificific PlPlatate 4.6 cm/yry 5.45.4 cm/ycm/yr Aleutian megat0 200200 B 150150° km 141400° associated with the local northeast-trending folds. More complex joint and vein in the Jurassic strata of the CIB might have contributed to the migration of hy- networks were later identified on the Insikin Peninsula by Gillis et al. (2013) and drocarbons from their Jurassic source rocks to Cenozoic sandstone reservoirs, Rosenthal et al. (2015), who noted that quartz and calcite cement is preserved and that the Jurassic strata may be a productive fractured reservoir (see also on the joint faces and that they are parallel to pervasive vein sets that are also Magoon and Claypool, 1981; Fisher and Magoon, 1982). cemented with quartz and calcite, as well as clay; they all suggested that the In this study we document orientations, distributions, and relative ages of joints and veins may be important with respect to CIB reservoir potential. Oil several regionally prominent joint and vein sets (Fig. 4) through field and re- seeps along regional joint sets have long been noted (Detterman and Hartsock, mote sensing-based observations. We present the first detailed investigation 1966). Helmold (2013) and LePain et al. (2013) suggested that fracture networks of the pervasive joint and vein networks that formed within Jurassic to late GEOSPHERE | Volume 14 | Number 1 Rosenthal et al. | Fracturing in the lower Cook Inlet basin, southern Alaska Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/14/1/23/4048404/23.pdf 24 by guest on 03 October 2021 Research Paper 165° 155° 145° 65° N 65° 62° 149° Anchorage t 60° 60° 151° y MaMapp area Area Castle Mountain Faul Matanuska Valle 155° 145° 55° Anchorage 153° 61° 61° n YaY t si akutat microplateApproximate slab edge a kutat mic t B ropla Lake Clark Faul Fault U.