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The Birth of a Forearc: the Basal Great Valley Group, California, USA Devon A https://doi.org/10.1130/G46283.1 Manuscript received 21 December 2018 Revised manuscript received 15 May 2019 Manuscript accepted 17 May 2019 © 2019 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 6 June 2019 The birth of a forearc: The basal Great Valley Group, California, USA Devon A. Orme1 and Kathleen D. Surpless2 1Department of Earth Sciences, Montana State University, P.O Box 173480, Bozeman, Montana 59717, USA 2Department of Geosciences, Trinity University, One Trinity Place, #45, San Antonio, Texas 78212, USA ABSTRACT accumulated unconformably on ophiolitic base- The Great Valley basin of California (USA) is an archetypal forearc basin, yet the tim- ment, broadly termed the Coast Range ophiolite ing, structural style, and location of basin development remain controversial. Eighteen of 20 (CRO), which lies structurally above the Fran- detrital zircon samples (3711 new U-Pb ages) from basal strata of the Great Valley forearc ciscan complex (Bailey et al., 1970). The tim- basin contain Cretaceous grains, with nine samples yielding statistically robust Cretaceous ing of the onset of Farallon subduction beneath maximum depositional ages (MDAs), two with MDAs that overlap the Jurassic-Cretaceous western North America and the tectonic origin of boundary, suggesting earliest Cretaceous deposition, and nine with Jurassic MDAs consistent the CRO are controversial (e.g., Dickinson et al., with latest Jurassic deposition. In addition, the pre-Mesozoic age populations of our samples 1996). In one model, eastward subduction of the are consistent with central North America sources and do not require a southern provenance. Farallon plate beneath North America within a We interpret that diachronous initiation of sedimentation reflects the growth of isolated depo- two-plate system was active by 180–165 Ma centers, consistent with an extensional model for the early stages of forearc basin development. (Wakabayashi, 1992; Mulcahy et al., 2018), and the CRO formed during extension in the forearc INTRODUCTION 1970). Eighteen (18) of our 20 samples contain region of an east-dipping Franciscan subduc- Forearc basins occupy a critical tectonic zone Cretaceous zircons, with nine samples yielding tion zone 172–164 Ma (Saleeby, 1996; Sher- above subducting plates, and their strata contain statistically robust Cretaceous maximum depo- vais, 2001). This subduction margin remained a record of subduction-related orogenesis (e.g., sitional ages (MDAs). As first noted by Surpless primarily non-accretionary for its first ~50 m.y. Dickinson, 1995; Hessler and Sharman, 2018). et al. (2006), a Cretaceous age revision for the and became strongly accretionary at ca. 123 Ma However, these basins have low preservation basal GVG: (1) lengthens the time interval (Dumitru et al., 2010; Wakabayashi, 2015). potential due to active-margin shortening and/or between initiation of subduction and onset of In an arc-arc collisional model, the Smartville destructive phases (e.g., Fildani et al., 2008), and forearc basin sedimentation; (2) lengthens the and Great Valley ophiolite segments of the CRO therefore, the mechanisms of initial basin forma- duration of the unconformity between the GVG formed as backarc ophiolites atop a west-dipping tion are not well understood. The Great Valley and its underlying basement; and (3) doubles subduction zone offshore western North America basin of California (USA) has been the focus of the thickness of Lower Cretaceous GVG strata. (Schweickert and Cowan, 1975; Ingersoll, 2000, more than 100 years of exploration, including Our results document diachronous accumu- 2019; Schweickert, 2015). These ophiolites and detrital zircon (DZ) provenance studies that have lation in the earliest Great Valley forearc region, island arcs accreted onto the California margin revealed sediment dispersal patterns (DeGraaff- with sedimentation beginning in either Late Ju- during the Sierran phase of the Nevadan orog- Surpless et al., 2002; Dumitru et al., 2012; Shar- rassic or Early Cretaceous time along the length eny ca. 162–155 Ma (e.g., Ingersoll, 2008). By man et al., 2015), but also called into question of the Sacramento Valley forearc basin. We sug- 150 Ma, the margin was a consolidated two-plate stratigraphic age constraints on the timing of ini- gest that initial Great Valley forearc sedimenta- system with eastward subduction generating arc tial basin sedimentation (Surpless et al., 2006). tion occurred in isolated latest Jurassic–earliest magmatism and initiation of sedimentation of the Uncertainty regarding the age of the basal Great Cretaceous sub-basins that overfilled to form a GVG atop the Great Valley ophiolite. Valley Group (GVG) impedes our understand- larger, single forearc basin during Early Cre- In both models, the provenance of the GVG ing of how the incipient forearc basin developed taceous time (DeGraaff-Surpless et al., 2002). is constrained to the Klamath-Sierran magmatic as the west coast of North America became a Documenting the birth of this ancient forearc ba- arc by sandstone petrography (e.g., Ingersoll, consolidated, two-plate subduction system (e.g., sin permits improved understanding of the early 1983), DZ geochronology (e.g., DeGraaff- Ernst, 1970; Dickinson, 1995). Here we provide stages of forearc basin development as well as Surpless et al., 2002), isotopic analysis (e.g., a record of the initiation and provenance of sedi- the Mesozoic development of the central-west- Linn et al., 1992), paleocurrent analysis (e.g., mentation within this archetypal forearc basin. ern margin of North America. Ingersoll, 1979), and mudstone geochemistry We revisit the timing of the earliest GVG (Surpless, 2014). In contrast, a translational sedimentation using U-Pb geochronology of GEOLOGIC SETTING model places basal GVG deposition south of 20 new DZ samples collected from basal GVG The Great Valley forearc basin developed be- the Sierra Nevada, with postulated northward strata (Fig. 1). All samples were collected from tween the Franciscan subduction complex to the translation to its current position west of the strata mapped as Upper Jurassic based on bio- west and the Sierra Nevada magmatic arc to the Sierran arc complete by ca. 120 Ma (Wright stratigraphy (Jones et al., 1969; Imlay and Jones, east (Fig. 1; Dickinson, 1995). Sediment initially and Wyld, 2007). CITATION: Orme, D.A., and Surpless, K.D., 2019, The birth of a forearc: The basal Great Valley Group, California, USA: Geology, v. 47, p. 757–761, https:// doi .org /10 .1130 /G46283.1 Geological Society of America | GEOLOGY | Volume 47 | Number 8 | www.gsapubs.org 757 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/47/8/757/4793706/757.pdf by guest on 01 October 2021 40º30'N A 122º45'W 122º30'W B 140.32 ± 0.66 (3) PH13 0510 15 20 n = 29/60 n = 31/60 Kilometers 0510 15 20 145.88 ± 0.86 (6) PH11 k Miles n = 28/51 n = 23/51 140.76 ± 0.66 (12) PH15 Dry Creek n = 48/60 n = 12/60 COLD FORK FAULT ZONE 153.34 ± 0.97 (6) PH08 Red Bluff n = 23/58 n = 35/58 Elder Cree 140.73 ± 0.96 (5) PH02 n = 32/55 n = 23/55 ELDER CREEK 141.71 ± 0.71 (16) DO61217-9 FAULT ZONE Elder Creek 40ºN n = 112/298 n = 186/298 Corning 151.76 ± 0.84 (4) JR09 McCarty Creek n = 17/30 n = 13/30 PASKENTA FAULT ZONE 148.70 ± 0.77 (10) JR03 n = 42/55 n = 13/55 Stony Creek Orland 145.82 ± 0.38 (17) DO61217-4 Grindstone n = 143/291 n = 148/291 Creek N 147.88 ± 0.39 (14) DO61217-8 n = 91/280 n = 189/280 Stream 142.88 ± 0.82 (4) JR07 Elk Creek Grindstone Creek Contact Willows n = 55/64 n = 9/64 Fault 39º30'N 144.37 ± 0.57 (8) DO61217-3 DZ sample n = 114/283 n = 169/283 locality from Stonyford Surpless et al. 148.00 ± 0.38 (20) DO61217-7 (2006) Funks n = 102/287 n = 185/287 DZ sample Creek locality, this study 148.90 ± 0.72 (14) JR17 Sites Stonyford Ophiolite Lodoga n = 41/60 n = 19/60 sample locality, this study 144.04 ± 0.24 (12) DO61517-2 Town Leesville n = 314/314 n = 0/314 149.14 ± 0.38 (20) DO61417-4 n = 150/295 n = 145/295 148.49 ± 0.32 (30) DO61417-3 Wilbur n = 177/297 n = 120/297 Springs 39ºN 135.44 ± 0.19 (63) DO61417-7 Tertiary-Quaternary rocks, n =304/311 n = 7/311 Franciscan Complex, 142.95 ± 0.45 (10) DO61417-6 Klamath terranes Cache n = 158/290 n =132/290 Coast Range ophiolite Creek McLaughlin Reserve 147.76 ± 0.59 (12) DO61417-2 Upper Cretaceous Great Valley Group McLaughlin n = 111/295 Reserve n = 184/295 Lower Cretaceous Great Valley Group 100140 180220 250 1000 2000 3000 Upper Jurassic Great Valley Group Age (Ma)Age (Ma) Figure 1. A: Geologic map showing sample locations within mapped “Jurassic” Great Valley Group strata, California, USA (modified from Surpless et al., 2006). DZ—detrital zircon. B: Probability distribution plots (PDPs) with YC2σ(3+) (youngest cluster of three or more grains that overlap within 2σ uncertainty) maximum depositional ages (MDAs); number of grains used to calculate MDAs is given in parentheses. See text for discussion of MDA calculation. n is the subset of U-Pb ages in each plot shown as a function of the total number of U-Pb ages in the sample. Samples are arranged in stratigraphic order within each region, with oldest sample on bottom. Colors on PDPs correspond to geologic periods: green—Cretaceous; light blue—Jurassic; magenta—Triassic; dark purple—Paleozoic; pink—Precambrian. BASAL STRATIGRAPHIC AGE calibration of the ammonite stratigraphy with the Franciscan Complex were interpreted as re- The Late Jurassic age assignment of the Lower Cretaceous calcareous nannofossils and worked deposits within Cretaceous strata, based basal GVG is based on two Tithonian zones interbedded radiometrically dated tuff hori- on abundant Cretaceous DZs (Dumitru et al., of the pelecypod Buchia (B.
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