Geologic map of southern Alaska showing major accreted terranes and fault systems, including the Yakutat terrane (modified after Plafker 1987; Plafker et al. 1994). Recent plate velocities between the Pacific plate and the Yakutat terrane are based on GPS measurements reported by Fletcher and Freymueller (1999). CR , Copper River; DRZ , Dangerous River Zone; IB , Icy Bay; KIZ , Kayak Island Zone; PZ , Pamplona Zone; YB , Yakutat Bay. The black rectangle indicates the location of this study. 

Contexts in source publication

Context 1

... Alaska has been the site of the collision of a number of far-traveled allochthonous terranes during the Cenozoic (Plafker 1987; Plafker et al. 1994). One of these, the Yakutat terrane, is cur- rently attached to the Pacific plate and is being underthrust beneath the Chugach terrane at rates of ∼ 0 : 56 mm = yr (Fletcher and Freymueller 1999; Veenstra et al. 2006). This terrane was transported northwestward, parallel to the Alaskan continental margin ( fig. 1). Two hypotheses have been offered to account for the Yakutat terrane ’ s transport history from ∼ 50 Ma to the present (cf. Bruns 1983; Plafker 1987). The first, here termed the northern option or the short-transport hypothesis, involves trans- port over a relatively short distance ( ∼ 600 km) from a northern position, with sedimentary cover strata being derived from local sources. This hypothesis invokes modest Neogene transport and significant shortening within the terrane boundary (Plafker et al. 1994). The second, here termed the southern option or the long-transport hypothesis, involves transport from a faraway southern position. This hypothesis is based on the reconstruction of mag- netic anomalies and the development of the subduction of the Kula-Farallon spreading center (Bruns 1983). The southern option places the basement rocks of the Yakutat terrane as far south as northern California or southern Oregon in the Eocene ( ∼ 45 Ma; cf. Bruns 1983; Plafker et al. 1994) and thus involves ∼ 1500 – 2000 km of northward transport along the Cordilleran margin. The sedimentary cover rocks would consequently reflect movement along the western edge of the northern Cordillera (see Cowan et al. 1997). The transport and accretion history of a terrane can be addressed by provenance analysis (i.e., Garver and Brandon 1994 b ; Cowan et al. 1997). By resolving the provenance of the sedimentary cover of the Yakutat terrane, we can identify different source areas and its transport history can be reconstructed. The sedimentary cover strata of the Yakutat terrane are well suited for zircon analysis, but so far few detrital zircon fission-track (DZFT) studies have been conducted (Armstrong 1988; Plafker et al. 1992; Johnston 2005; Meigs et al. 2008) and virtually no detrital zircon U/Pb analyses have been done on these rocks (see Gehrels et al. 1995; Haeussler et al. 2006). In contrast, a number of studies have characterized the various plutonic and volcanic rocks of the northern Cordillera from the southwestern coast of Oregon and Washington to the Chugach/St. Elias Range in southern/southeastern Alaska (Anderson 1988; Garver and Brandon 1994 a , 1994 b ; Bradley et al. 2003; Himmelberg et al. 2004). The most tectonically significant element of the continental margin is the Coast Plutonic Complex (CPC) that extends from northwestern Washington into southeastern Alaska (Armstrong 1988). Intrusion and exhumation of this complex have been dated from ∼ 80 to 45 Ma (Harrison et al. 1979; Parrish 1983; Armstrong 1988; Armstrong and Ward 1991; Stowell and Crawford 2000). The CPC plays a key role in the reconstruction of Yakutat terrane transport history because in either a long- or a short- transport hypothesis, the CPC must have been a source to the Yakutat cover sequence. In this study, 13 sandstones from the sedimentary cover of the Yakutat terrane were analyzed for their petrography. Then detrital zircons from these sandstones were analyzed by fission track (FT) to obtain cooling ages, followed by U/Pb dating of each of the same grains to determine its crystallization age (fig. 2). This double-dating distinguishes between zircons derived from volcanic sources (where single grains have identical FT and U/Pb ages) and those derived from plutonic and metamorphic sources (Carter and Moss 1999; Carter and Bristow 2003). Afterward, vitrinite reflectance on isolated kerogen was analyzed for four samples to establish the maximum postdepositional temperature range. The analytical data presented here are interpreted in relation to stratigraphic age and significance for transport of the Yakutat terrane. The Yakutat terrane is approximately 600 km long and 200 km wide. Its transport and collision have resulted in the transition of the Queen Charlotte – Fairweather transcurrent fault in the east to the Alaska-Aleutian subduction zone in the west. Geophysical, seismic, and structural studies show that the Yakutat terrane arrived at its present position along the southern Alaska margin since the Pliocene (Bruns 1983; Fletcher and Freymueller 1999, 2003). GPS measurements of plate velocity for the Yakutat microplate indicate current plate motion of ∼ 44 mm = yr to the north (Plafker et al. 1994; Fletcher and Freymueller 1999, 2003), a rate slightly slower than Pacific plate motion of ∼ 52 mm = yr in the region (Sauber et al. 1997; Fletcher and Freymueller 1999, 2003). Successive accretion of the Chugach – Prince William (CPW) composite terrane to the north of the Yakutat terrane occurred from Late Cretaceous to early Tertiary time (Plafker 1987; Plafker et al. 1994). It is inferred that the Chugach terrane was accreted before the Prince William terrane and formed a backstop for collision of outboard terranes (Plafker et al. 1994). Rocks of the Chugach terrane are locally metamorphosed up to amphibolite facies, and this metamorphism is inferred to be the result of a northwestward shift in Kula plate motion and/or the ...

Context 2

... of a terrane can be addressed by provenance analysis (i.e., Garver and Brandon 1994 b ; Cowan et al. 1997). By resolving the provenance of the sedimentary cover of the Yakutat terrane, we can identify different source areas and its transport history can be reconstructed. The sedimentary cover strata of the Yakutat terrane are well suited for zircon analysis, but so far few detrital zircon fission-track (DZFT) studies have been conducted (Armstrong 1988; Plafker et al. 1992; Johnston 2005; Meigs et al. 2008) and virtually no detrital zircon U/Pb analyses have been done on these rocks (see Gehrels et al. 1995; Haeussler et al. 2006). In contrast, a number of studies have characterized the various plutonic and volcanic rocks of the northern Cordillera from the southwestern coast of Oregon and Washington to the Chugach/St. Elias Range in southern/southeastern Alaska (Anderson 1988; Garver and Brandon 1994 a , 1994 b ; Bradley et al. 2003; Himmelberg et al. 2004). The most tectonically significant element of the continental margin is the Coast Plutonic Complex (CPC) that extends from northwestern Washington into southeastern Alaska (Armstrong 1988). Intrusion and exhumation of this complex have been dated from ∼ 80 to 45 Ma (Harrison et al. 1979; Parrish 1983; Armstrong 1988; Armstrong and Ward 1991; Stowell and Crawford 2000). The CPC plays a key role in the reconstruction of Yakutat terrane transport history because in either a long- or a short- transport hypothesis, the CPC must have been a source to the Yakutat cover sequence. In this study, 13 sandstones from the sedimentary cover of the Yakutat terrane were analyzed for their petrography. Then detrital zircons from these sandstones were analyzed by fission track (FT) to obtain cooling ages, followed by U/Pb dating of each of the same grains to determine its crystallization age (fig. 2). This double-dating distinguishes between zircons derived from volcanic sources (where single grains have identical FT and U/Pb ages) and those derived from plutonic and metamorphic sources (Carter and Moss 1999; Carter and Bristow 2003). Afterward, vitrinite reflectance on isolated kerogen was analyzed for four samples to establish the maximum postdepositional temperature range. The analytical data presented here are interpreted in relation to stratigraphic age and significance for transport of the Yakutat terrane. The Yakutat terrane is approximately 600 km long and 200 km wide. Its transport and collision have resulted in the transition of the Queen Charlotte – Fairweather transcurrent fault in the east to the Alaska-Aleutian subduction zone in the west. Geophysical, seismic, and structural studies show that the Yakutat terrane arrived at its present position along the southern Alaska margin since the Pliocene (Bruns 1983; Fletcher and Freymueller 1999, 2003). GPS measurements of plate velocity for the Yakutat microplate indicate current plate motion of ∼ 44 mm = yr to the north (Plafker et al. 1994; Fletcher and Freymueller 1999, 2003), a rate slightly slower than Pacific plate motion of ∼ 52 mm = yr in the region (Sauber et al. 1997; Fletcher and Freymueller 1999, 2003). Successive accretion of the Chugach – Prince William (CPW) composite terrane to the north of the Yakutat terrane occurred from Late Cretaceous to early Tertiary time (Plafker 1987; Plafker et al. 1994). It is inferred that the Chugach terrane was accreted before the Prince William terrane and formed a backstop for collision of outboard terranes (Plafker et al. 1994). Rocks of the Chugach terrane are locally metamorphosed up to amphibolite facies, and this metamorphism is inferred to be the result of a northwestward shift in Kula plate motion and/or the subduction of the Kula-Farallon spreading center beneath the accreted terrane from the Late Paleocene to the Middle Miocene (Engebretson et al. 1985; Plafker 1987; Lonsdale 1988; Stock and Molnar 1988; Atwater and Stock 1998). Subduction of this spreading center resulted in the intrusion of dikes and plutons at ∼ 65 – 50 Ma, and these form the time-trangressive Sanak-Baranof plutonic belt (Plafker et al. 1994; Bradley et al. 2003; Trop and Ridgway 2007). Underthrusting of ∼ 225 km of the Yakutat terrane beneath the Chugach terrane has resulted in the formation of the Chugach/St. Elias Range. This collision forced dramatic surface uplift and ero- sional exhumation (Plafker 1987; Meigs and Sauber 2000; Montgomery 2002; Spotila et al. 2004; Berger and Spotila 2008; Berger et al. 2008). The stratigraphy of the Yakutat terrane records sedimentation since the Eocene (Plafker 1987). The sedimentary cover is underlain by two types of basement: (1) poorly studied ∼ 50 – 55-Ma basaltic oceanic plateau rocks to the west and (2) continental margin rocks of the Upper Paleocene and Lower to Middle Eocene Orca Group to the east, including metamorphosed rocks of the Chugach terrane (Plafker 1987). The basement units are sep- arated by the inactive Dangerous River Zone (fig. 1), a north-south-striking high-angle reverse fault (Plafker 1987; Bruhn et al. 2004) that marks an internal transition from metamorphosed Chugach continental crust to oceanic plateau. Stratigraphic sequences vary in facies and thicknesses across this boundary, but only strata west of the DRZ are analyzed and discussed in this article. In the eastern part of our study area, the Yakutat cover strata rest unconformably on the Orca Group, part of the CPW terrane (Plafker et al. 1994). Lying unconformably on the Orca Group are three formations that include the Middle Eocene Kulthieth Formation ( ∼ 55 : 8 – 33 : 9 Ma), the Lower Oligocene to Lower Miocene Poul Creek Formation ( ∼ 33 : 9 – 23 : 0 Ma), and the Miocene to Pleistocene Yakataga Formation ( ∼ 23 : 0 Ma to Present; fig. 2; Plafker 1987). Age estimates are based on Gradstein et al. (2004). Kulthieth Formation. The Kulthieth Formation consists mainly of organic-rich sandstones that unconformably overlie the Orca Group in the northern Robinson Mountains. This unit consists of arkosic sandstone with interbedded medium- to thick-bedded coal seams and abundant marine fossils (Plafker 1987). Biostratigraphic ages of the fossil assemblages suggest deposition during the Early Eocene to the Early Oligocene ( ∼ 54 – 33 Ma) in nonmarine alluvial-plain, delta-plain, barrier- beach, and shallow-marine settings (Plafker 1987). Sandstones contain abundant cross-bedding, showing sediment transport to the northwest (Plafker 1987; Bruhn et al. 2004). The estimated thickness of the Kulthieth Formation in the Yakataga area is ∼ 2 : 8 km (Miller 1957, 1971; Plafker 1987; Wahrhaftig et al. 1994), but the depositional age range for this formation is not well constrained. Poul Creek Formation. The Poul Creek Formation conformably overlies the Kulthieth Formation and consists of highly deformed, fine-grained, glau- conitic, organic-rich sandstones interbedded with water-lain basaltic tuff, breccia, pillow lava, and related deposits of the intertonguing Cenotaph Volcanics (Plafker 1987). The Poul Creek Formation records a marine transgression and has an estimated thickness of ∼ 1 : 9 km (Plafker et al. 1980; Plafker 1987). Age constraints for this unit are better than those for the Kulthieth Formation, ranging from Upper Eocene to Lower Oligocene and possibly into the Lower Miocene ( ∼ 40 – 20 Ma; Plafker 1987). DZFT data with young grain-age populations of ∼ 24 to ∼ 29 Ma help constrain depositional age (Johnston 2005). The nonmarine Cenotaph Volcanics and marine Topsy Formation to the east are believed to be post – Lower Oligocene to pre – Middle Miocene in age and range in thickness from ∼ 0 : 5 to 1.5 km (MacKevett et al. 1971). Yakataga Formation. The Miocene to Plio- Pleistocene Yakataga Formation overlies the Poul Creek Formation and consists mainly of glacially derived sediment with abundant marine strata con- taining dropstones and diamictites. The unit thick- ens from east to west and locally reaches ∼ 6 km total (Bruns and Schwab 1983; Plafker 1987). The formation is generally divided into an upper and a lower unit. Deposition of the lower unit occurred in neritic to bathyal water depths (Plafker 1987; Martin 1993). The upper unit is recognized by its transition into thick diamictite characterized by dropstones in bioturbated sandstones inferred to have been derived from the Chugach and St. Elias mountains (Eyles and Lagoe 1990; Lagoe et al. 1993; Martin 1993; Lagoe and Zellers 1996; White et al. 1997). Deposition of the Yakataga Formation appar- ently records the onset of late Cenozoic glaciation and includes the development of high-latitude ice sheets in the Northern Hemisphere at ∼ 5 – 6 Ma (Kennett 1986; Eyles and Lagoe 1990). Depositional age constraints, which are poor, are based on benthic foraminifera and molluscan fauna and range from Miocene to Plio-Pleistocene (Plafker 1987; Eyles and Lagoe 1990). Thirteen sandstone samples were analyzed with FT and U/Pb double dating of single zircons. For DZFT analysis, zircon aliquots of each sample were mounted in Teflon and polished to expose internal crystal surfaces etched in a NaOH-KOH eutectic melt (see Bernet and Garver 2005). The resulting composite probability distribution was deconvolved into component peak ages for each sample, and then all samples from each of the three formations were combined (figs. 3, 4; Brandon and Vance 1992; Brandon 1996; see details in Perry 2006). U-Pb analysis on two samples from each of the three formations was done with a Micromass Iso- Probe multicollector inductively coupled mass spectrometer (ICPMS) with a laser ablation system at the University of Arizona, Tucson. For each sample all FT-dated grains were dated by U/Pb analysis ( ∼ 650 double-dated grains); then an additional 50 randomly selected grains from the formation were analyzed (150 grains per formation total; fig. 5). Standard sedimentary petrologic analysis was done on thin sections of all DZFT-dated ...