Upper plate proxies for flat-slab subduction processes in southern Alaska

Earth and Planetary Science Letters 303 (2011) 348–360

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Earth and Planetary Science Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e p s l

Upper plate proxies for flat-slab subduction processes in southern Alaska Emily S. Finzel a,⁎, Jeffrey M. Trop b, Kenneth D. Ridgway a, Eva Enkelmann c a b c

Dept. of Earth and Atmospheric Sciences, Purdue University, West Lafayette, IN 47907, USA Dept. of Geology, Bucknell University, Moore Avenue, Lewisburg, PA 17837, USA Universität Tübingen-Institut für Geowissenschaften, Wilhelmstr. 56, 72074, Tübingen, Germany

a r t i c l e

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Article history: Received 19 June 2010 Received in revised form 12 January 2011 Accepted 13 January 2011 Available online 12 February 2011 Editor: T.M. Harrison Keywords: Alaska flat-slab subduction geochronology thermochronology provenance sedimentary basin

a b s t r a c t The timing of initiation of flat-slab subduction beneath southern Alaska and the upper plate record of this process are not well understood. We explore the record of flat-slab subduction in southern Alaska by integrating stratigraphic, provenance, geochronologic, and thermochronologic data from the region directly above and around the perimeter of ongoing flat-slab subduction. These datasets document a change from regional Paleocene–Oligocene subduction-related magmatism and basin development to an absence of magmatism and initiation of rock exhumation that continues to today. We infer that initiation of flat-slab subduction prompted crustal shortening, exhumation, inversion of sedimentary basins, and cessation of magmatism above and around the area of ongoing flat-slab subduction. Surface uplift and erosion above the flat slab resulted in deposition of thick, clastic wedges in sedimentary basins located along the western and northern perimeters of the flat-slab region. Along the eastern perimeter, northwestward-propagating Oligocene–Quaternary slab-edge volcanism and transtensional basin development along dextral strike-slip faults record progressive northwestward insertion of a shallow slab against the curved continental margin of eastern Alaska. Collectively, these geologic data indicate that flat-slab subduction was shaping southern Alaska by late Eocene–early Oligocene time, much earlier than previous models infer. Upper plate processes related to subduction of a flat slab in Alaska are similar to those documented in other modern flat-slab regions. These processes include: 1) shortening and exhumation of the upper plate several hundred kilometers inboard from the plate margin, 2) cessation of subduction-related magmatism within ten million years of the onset of shallow subduction, 3) shoaling or inversion of sedimentary basins above the flat-slab region, 4) deformation and/or erosion of the accretionary prism during flat-slab subduction, and 5) in some settings, reestablishment of sedimentation in the accretionary prism after the shallow slab has migrated laterally. Unlike other flat-slab margins, strike-slip-related volcanism and basin development characterizes one edge of the flat-slab region in Alaska, a consequence of oblique insertion of the flat slab into the corner of a curved continental margin. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The upper plate geologic record of flat-slab subduction is a topic of considerable discussion for both modern and ancient convergent margins (e.g., Dickinson and Snyder, 1979; Espurt et al., 2008; Hampel, 2002; Jordan and Allmendinger, 1986). Studies based on seismicity and associated crustal deformation suggest that flat-slab subduction results in zones of broad, diffuse deformation at convergent plate margins as it increases the transfer of compressive stresses inboard and upward to the overriding plate (Gutscher et al., 2000; Lallemand et al., 2005). The southern margin of Alaska is characterized by an east-to-west transition from transform tectonics to flat-slab subduction to normal subduction (Figs. 1 and 2). On the upper plate, the eastern region is marked by active volcanism in the ⁎ Corresponding author. Now at ExxonMobil Exploration Co., Houston, TX 77060, USA. Tel.: + 1 765 490 9055; fax: + 1 765 496 1210. E-mail address: emily.s.fi[email protected] (E.S. Finzel). 0012-821X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2011.01.014

Wrangell volcanic belt, active dextral displacement along the Denali– Totschunda fault system, and shallow seismicity (b50 km; Figs. 1 and 3A). The central region is distinguished by relatively high topography, a lack of active volcanism, and a shallowly-dipping Wadati–Benioff zone produced by the Yakutat microplate that extends nearly horizontally for ~ 250 km northwestward beneath Alaska before reaching a depth of 150 km more than ~ 600 km inboard of the Aleutian trench (Figs. 2 and 3B). In the western region, subduction of the Pacific oceanic plate produces a more steeply dipping Wadati– Benioff zone that reaches depths of 100–150 km within ~400 km of the trench (Figs. 2 and 3C) and active volcanism in the Aleutian arc (AVA on Figs. 1 and 3C). Cenozoic deformation in southern Alaska associated with subduction of the Yakutat microplate was first proposed by Plafker et al. (1978) and expanded on by Plafker (1987) and Plafker et al. (1994). In these previous models, normal oceanic crust of the northwestern part of the Yakutat microplate was subducted beneath Alaska from early Oligocene (ca. 30 Ma) to middle Miocene time; the onset of subduction was based

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Fig. 1. Tectonic framework of southern Alaska, including faults with known or suspected Neogene and younger displacement (modified from Plafker et al., 1994), Holocene volcanoes (red triangles; modified from Cameron, 2005), plate motion vectors relative to North America (DeMets et al., 1994; Leonard et al., 2007), exposed (light yellow) and interpreted subducted extent of the Yakutat microplate (region within bold dashed line), locations of exhumation studies discussed in Section 2.1 (yellow circles) and Section 4.2 (orange circles), and locations of U-Pb detrital samples discussed in Section 3.1 (blue rectangles). The flat-slab region is the area within the solid red line. Edges of the subducted Yakutat microplate are loosely constrained (modified from Eberhart-Philips et al., 2006 and Fuis et al., 2008). Width of the Neogene accretionary prism shown in gray shaded polygon (modified from vonHuene et al., 1999). Sedimentary basins: CI, Cook Inlet; TB—Tanana, CB—Colorado Creek, MB—Matanuska, SB—Susitna, CR—Copper River. AVA—Alaska PeninsulaAleutian volcanic arc; MD-Mount Drum; MC-Mount Churchill; KP—Kenai Peninsula; PWS—Prince William Sound; WAR—western Alaska Range; CAR—central Alaska Range; EAR— eastern Alaska Range; BM, Buzzard Creek maar; WVB—Wrangell volcanic belt; CMF—Castle Mountain fault; CSEF—Chugach-St. Elias fault; DRF-Duke River fault; QC-FF.—QueenCharlotte-Fairweather fault; TF—Totschunda fault.

on the age of the oldest lavas in the Wrangell volcanic belt (WVB on Fig. 1). These previous models infer that beginning in middle Miocene time, more buoyant continentalized crust of the southeastern part of the Yakutat microplate was subducted at a shallow angle beneath the continental margin of south-central Alaska based on the Neogene age of a N5-km-thick succession of siliciclastic strata (i.e., the Yakataga Formation) deposited upon the unsubducted part of the Yakutat microplate (Plafker et al., 1994). Recent geophysical studies, however, have shown that the entire Yakutat microplate (both the subducted and unsubducted parts) consists of thick, buoyant crust (Christeson et al., 2010; EberhartPhilips et al., 2006; Ferris et al., 2003; Lowe et al., 2008). The subducted Yakutat slab is imaged as a low-velocity zone with a high ratio of P-wave to S-wave velocities, consistent with the typical seismic character of thick oceanic crust. Compilation of these recent studies shows that the crust of the subducted northwestern portion of the Yakutat microplate is 11–22-km-thick and the crust of the unsubducted southeastern portion is 20–25 km thick. Contemporary studies have focused on the timing for initiation of subduction of the thicker southeastern portion of the microplate. These studies support a middle–late Miocene timing for initiation of flat-slab subduction based on the creation of high topographic relief in the modern

Chugach, St. Elias, and Talkeetna Mountains (Hoffman and Armstrong, 2006; O'Sullivan and Currie, 1996; Parry et al., 2001), and the imaged length of the subducted portion of the Yakutat microplate compared to present-day plate motions (Koons et al., 2010). In all these models, flat-slab subduction in southern Alaska represents a relatively young event that initiated since middle Miocene time, an interpretation that has been propagated through the literature (i.e., Gulick et al., 2007; Pavlis et al., 2004; Rondenay et al., 2010). In light of the new geophysical findings indicating that the entire Yakutat slab has thick crust, our study reviews previously published geologic data and presents new provenance data from the upper plate of southern Alaska. All these datasets are evaluated in the context of flat-slab processes. In particular, our documentation of magmatism, exhumation, basin development, and provenance provides an integrated record of upper plate responses to flat-slab subduction and evidence that flat-slab subduction began as early as late Eocene– early Oligocene time. 2. Processes above flat-slab region The area above the present-day flat-slab subduction in southern Alaska is characterized by large-magnitude earthquakes, Oligocene or

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Fig. 2. Map of southern Alaska illustrating slab seismicity (N 50 km depth; Alaska Earthquake Information Center catalog) and locations of transects shown in Fig. 3. Additional symbols are the same as in Fig. 1. Note the northeastward increase in the gap between slab seismicity and the trench as well as the paucity of seismicity deeper than 50 km along the northeastern edge of the slab.

older bedrock exposures, no Quaternary volcanoes, and transpressive deformation evidenced by folds, reverse faults, and dextral strike-slip faults. The flat-slab region is defined as the area of the upper plate presently above the shallow part of the subducted Yakutat slab (b50 km deep; region within solid red line on Fig. 1). Integration of

Fig. 3. Cross-sections showing changes in seismicity (within ~ 50 km of each transect) between eastern, central, and western transects across southern Alaska (Alaska Earthquake Information Center catalog). Locations of transects shown on Fig. 2. Note that seismicity from all depths is shown and transects are aligned parallel with presentday plate motions. DF—Denali fault; TR—Transition fault; see Fig. 1 for additional abbreviations. Default depths of 10 km and 33 km are assigned for events with poorly constrained depths in oceanic and continental areas, respectively.

new and recently published geochronologic, thermochronologic, and stratigraphic data help to constrain the timing of magmatism, exhumation, and basin development in the flat-slab region. 2.1. Geochronology 2.1.1. Data Although previous workers document the presence of a modern volcanic gap above the subducting flat-slab (Nye, 1999; Fuis et al., 2008), the timing of regional cessation of magmatism has not been reported in the literature. Our new compilation of all published Cenozoic crystallization ages from plutonic and volcanic rocks exposed in south-central Alaska indicates cessation of subduction-related magmatism in the flatslab region during late Paleogene time (Fig. 4). The publications from which the data were collected range from 1966 to 2007 and include data between 140°W–156°W longitude and 60°W–63°W latitude (region within dotted box on Fig. 1). The compilation includes 505 ages from bedrock samples analyzed using various techniques and phases (see Supplementary material for details). Ages that the original authors deemed questionable or reset and dates on air-fall material were excluded. Geochronologic data were not recalibrated using modern standards. Subduction-related magmatism in southern Alaska is attributed to various episodes of volcanic arc activity, as well as subduction of a spreading ridge between ca. 61 and 50 Ma that created a slab-window that allowed for mantle upwelling to the upper plate (see References in Supplementary material). Magmatism was regionally extensive across south-central Alaska during early Paleogene time (ca. 65.5– 44 Ma), but ceased east of ~148°W by ca. 43 Ma and west of ~ 148°W by ca. 32 Ma based on the published age data. 2.1.2. Interpretation The cessation of magmatism east of 148° was concurrent with major plate reorganization when relative plate convergence between

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Fig. 4. Plot comparing published igneous ages of plutonic and volcanic rocks versus longitude in south-central Alaska in area outlined by dotted box on Fig. 1. Dashed line denotes present region above flat-slab subduction. Each circle represents a calculated age that does not depict errors. Note cessation of magmatism throughout the flat-slab region east of ~ 148°W by ca. 43 Ma and west of ~ 148°W by ca. 32 Ma. See Data Repository for age data and references.

the subducting oceanic plate and North America shifted from N–NE to more NW-directed motion ca. 46–43 Ma (Doubrovine and Tarduno, 2008; Engebretson et al., 1985; Stock and Molnar, 1988). This shift in plate motion would be expected to prevent subduction-related magmatism east of ~ 148°W because that region would have been located inboard (east) of the new transform boundary developed as a consequence of the change in plate motions and therefore would not have a slab actively subducting beneath that part of the plate margin. Plafker (1987) proposed that the transform boundary originally coincided with the Transition fault (Fig. 1) and consequently shifted inboard (east) ca. 30 Ma, presumably to its present-day position along the Queen Charlotte–Fairweather fault system (Fig. 1), prior to the Yakutat microplate moving northward and subducting beneath Alaska. We agree that the paleo-transform boundary likely coincided with the present-day Transition fault that separates the Pacific and Yakutat plates, but we infer that its initiation occurred sometime between ca. 46 and 32 Ma. We interpret cessation of magmatism west of ~148°W as the product of shallow northwestward subduction of the Yakutat slab and associated compressive crustal stresses in the flat-slab region that would have inhibited the rise and eruption of magmas generated from heating and dehydration of the subducted slab (e.g., McNamara and Pasyanos, 2002; Nye et al., 2002; Qi et al., 2007). Subduction of oceanic lithosphere with thick crust (aseismic ridges, oceanic plateaus or seamount chains) typically coincides spatially and temporally with the absence of arc volcanism (e.g., McGeary et al., 1985; van Hunen et al., 2002). Localized conduits did permit isolated small-volume eruption of magmas along the leading Yakutat slab edge. For example, small Holocene maars in the foothills north of the Alaska Range (BM on Fig. 1) record eruption of subduction-related magmas far inboard along the northern edge of the subducted Yakutat slab (Nye et al., 2002).

2.2. Thermochronology 2.2.1. Data Compilation of all published low-temperature thermochronologic data from above the subducting Yakutat slab provides a record of the onset of rock exhumation in the upper plate. The publications from which the data were collected range from 1989 to 2010 and include all available thermochronologic data younger than 43 Ma from within the area above the subducted Yakutat slab (black dashed line on Fig. 1). Our analysis focuses on thermochronologic data that record exhumation after 43 Ma.

On the upper plate in the northern St. Elias Mountains, apatite and zircon fission track (FT) analyses record middle Eocene (ca. 43 Ma) initiation of rapid exhumation with distinct pulses of exhumation during late Eocene through Pliocene time (ca. 40–5 Ma), and peak exhumation ca. 35–20 Ma (1a on Fig. 1; Fig. 5; Enkelmann et al., 2008, 2010; O'Sullivan and Currie, 1996). In the Prince William Sound region to the west, apatite FT and (U-Th)/He ages reflect exhumation between ca. 35 and 3 Ma (2 on Fig. 1; Arkle et al., 2009). Farther northwest (~300 km) in the northern Chugach Mountains, detrital zircon FT data from near the Knik, Matanuska, and Tazlina glaciers record significant exhumation events at 40, 36, and 28 and 3.1 Ma, respectively (3a, 3b, and 3c on Fig. 1; Sendziak et al., 2009). In the same region, apatite FT ages suggest exhumation between ca. 24 and 17 Ma (4 on Fig. 1; Little and Naeser, 1989). North of the Chugach Mountains (U-Th)/He ages

Fig. 5. Histogram of cooling ages from bedrock and detrital samples collected in the St. Elias Mountains (1a, 1b on Fig. 1) in the southern part of the flat-slab region. Fission track ages are from detrital zircons collected in modern rivers that drain source terranes in the upper plate as well as accreted lower plate material. The bulk of zircon fission track (FT) ages that range from ca. 40 to 5 Ma (late Eocene–early Pliocene) are from rivers with source terranes on the upper plate. These ages suggest exhumation began in the southern part of the flat-slab region by late Eocene time. The Pliocene (b5 Ma) peaks in the bedrock apatite (U-Th)/He and detrital zircon FT cooling ages are from samples located on the south side of the St. Elias Mountains (1b on Fig. 1), where accretion of the Yakutat sedimentary cover is ongoing. Zircon FT cooling ages older than ca. 90 Ma are from samples located north of location 1a on Fig. 1. This study provides a new interpretation for these age data; see Berger and Spotila (2008), Berger et al. (2008), Enkelmann et al. (2008, 2009, 2010), and Spotila et al. (2004) for detailed explanations of sample collection and analyses.

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record exhumation in the Talkeetna Mountains (5 on Fig. 1) ca. 20– 15 Ma (Hoffman and Armstrong, 2006). In the eastern Alaska Range, ~50 km north of the flat-slab region (6 on Fig. 1), apatite FT ages range from 15 Ma to 1 Ma and (U-Th)/He cooling ages range from 4 Ma to 1 Ma (Benowitz et al., in press). Combined with higher-temperature thermochronologic ages from K-feldspar and biotite, the available data indicate rapid exhumation in the eastern Alaska Range from 22 Ma to present (Benowitz et al., in press). Apatite FT ages from Mt. McKinley in the central Alaska Range, located even farther north of the flat-slab region (~200 km; 7 on Fig. 1), indicate a sharp increase in exhumation ca. 6–5 Ma (Fitzgerald et al., 1995). The youngest apatite and zircon cooling ages found in Alaska today are located in the southern and central St. Elias Mountains in lower plate material that has been accreted to the upper plate. In this region, ongoing Yakutat subduction causes rapid rock uplift of the accreted material that is then efficiently eroded by massive glaciers (1b on Fig. 1; Fig. 5; Berger and Spotila, 2008; Berger et al., 2008; Enkelmann et al., 2009, 2010). 2.2.2. Interpretation The distinct episodes of exhumation in the upper plate described above were attributed by the original authors to changes in plate motions (ca. 43, 20, and 5 Ma), underplating of subducted Yakutat material (ca. 15 Ma), and collision of the thicker, southeastern part of the Yakutat microplate with southern Alaska (between ca. 25 and 10 Ma). While more low-temperature thermochronologic data are needed in southern Alaska to fully understand the spatial pattern of Cenozoic rock uplift, integration of the existing datasets clearly shows significant upper plate exhumation beginning in late Eocene time and continuing through Neogene time, as well as a potential general northwestward younging pattern of rock uplift and exhumation. Exhumation in the upper plate of the flat-slab region broadly overlapped with cessation of magmatism recorded by geochronometry (Fig. 4). 2.3. Stratigraphy 2.3.1. Data Stratigraphic data document a well-defined depositional hiatus across most of the flat-slab region during Miocene–Holocene time. Remnant sedimentary basins in the flat-slab region stopped accumulating clastic strata and were deformed by reverse faults and folds. For example, folded middle to late(?) Oligocene and older sedimentary basin strata in the Matanuska Valley region are unconformably overlain by Quaternary surficial deposits (Figs. 1, 6A and 7; Trop et al., 2003). North of the flat-slab region (~130 km), early Oligocene and older sedimentary strata along the south flank of the Alaska Range are unconformably overlain by Quaternary surficial deposits (CB on Figs. 1 and 7; Fig. 6B; Trop et al., 2004). 2.3.2. Interpretation We interpret this regional stratigraphic hiatus to represent late Oligocene and younger basin inversion associated with flat-slab subduction beneath Alaska. Folds and reverse faults regionally deform these Oligocene and older sedimentary strata (Kortyna et al., 2009; Pavlis and Roeske, 2007; Trop et al., 2004), consistent with Neogene contractile deformation coeval with basin inversion. Neogene shortening and inversion of basinal strata in the upper plate of the flat-slab region began shortly after cessation of magmatism recorded by geochronometry (Fig. 4) and rock uplift and erosion recorded by thermochronometry (Fig. 5). 3. Processes around flat-slab perimeter Sedimentary basins located along the western and northern perimeter of the flat-slab region record accumulation of thick, clastic wedges coeval with exhumation and basin inversion above the flat slab.

Integration of published stratigraphic and compositional data with new provenance data help link the depositional history of the perimeter basins to exhumation of the upper plate in the flat-slab region. Along the eastern perimeter, volcanic strata are the product of transtensional tectonics and partial melting of the eastern edge of the flat slab. 3.1. Western perimeter The Cook Inlet basin is located along the western perimeter of the flat-slab region (Fig. 1), where Cenozoic nonmarine depositional environments deposited up to 10 km of strata in parts of the basin (Flores et al., 2004). Paleogene strata (an unnamed unit, West Foreland and Hemlock Formations) reach a maximum combined thickness of ~ 2.5 km, whereas mainly Neogene strata (Tyonek, Beluga, and Sterling Formations) are much thicker (up to 7.5 km) and contain coal seams up to 15 m thick (Figs. 7 and 8A–B). Sediment accumulation rates increased beginning in late Oligocene time continuing through Pliocene time (Fig. 9). Interpretation of new provenance data suggests that Cook Inlet detritus was derived predominantly from source terranes located northeast of the basin above the flat-slab region and areas to the north in the central Alaska Range during Eocene through Pliocene time. The U-Pb ages of 590 detrital zircons from Eocene through Pliocene strata of the Cook Inlet basin contain significant middle Cretaceous (ca. 94– 111 Ma) and Late Cretaceous–Paleogene (ca. 47–72 Ma) age populations (Fig. 10). The low ratios of uranium–thorium (b10) in most of the grains (~ 98%; Fig. 10) suggest that they were derived from igneous rather than metamorphic source areas. The Late Cretaceous– Paleogene detrital ages match the age of plutons exposed in the western and central Alaska Range and the Talkeetna Mountains (Wilson et al., 1998; 2009). The greater regional thickness of the late Oligocene–early Miocene and late Miocene–Pliocene strata relative to the rest of the Cenozoic units, as well as the documentation of coeval exhumation events in the western and central Alaska Range, has previously been inferred to reflect regional exhumation and increased sediment influx from the central and western Alaska Range during these times (Fitzgerald et al., 1995; Haeussler, 2008; Kirschner and Lyon, 1973; Plafker et al., 1992). Comprising a conspicuously small population in our detrital data, however, are 30–45 Ma detrital zircon ages (gray bar on Fig. 10) that would match the ages of plutons that are common in the western Alaska Range (Moll-Stalcup, 1994; Wilson et al., 2009), precluding that region as a dominant sediment source. The dominant peak of Middle Cretaceous detrital zircon ages (Fig. 10) closely matches the ages of plutons that are widespread in the central Alaska Range and the Talkeetna Mountains, both located above the flat-slab region; similar age plutons are very sparsely scattered in the western Alaska Range (Wilson et al., 2009). The detrital populations also contain several distinct ages (such as ca. 210, 194, 156–176, 104– 143, 80, and 66–52 Ma in Fig. 10) that are interpreted to best reflect sediment input from source rocks in the flat-slab region. Key potential source areas for these populations in the flat-slab region include the Talkeetna arc and Alaska–Talkeetna Range belt in the Talkeetna Mountains, as well as the Chitina and Chisana arcs. Plutons representing the Mesozoic Chitina and Chisana arc systems are located in the eastern Alaska Range and eastern Chugach Mountains in the eastern part of the flat-slab region (Fig. 1; Miller, 1994; Nokleberg et al., 1994; Plafker et al., 1989; Roeske et al., 2003; Short et al., 2005; Snyder and Hart, 2005, 2007). Additional possible sources located above the flat-slab region that have ages that overlap with age populations in our samples include accretionary prism strata of the Valdez Group in the western Chugach Mountains (Bradley et al., 2009), and exhumed Cretaceous–Paleogene strata in the Matanuska Valley (Fig. 1; Kortyna et al., 2010; Trop, 2008). For more details on the ages and distribution of potential sources for Cenozoic sediment of the Cook Inlet basin see Finzel (2010). We suggest the existence of two major sediment pathways derived from above the flat-slab region based on our provenance data. One

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Fig. 6. Photos of Eocene–Oligocene sedimentary strata that record basin inversion and crustal shortening above the flat-slab region during Miocene–Holocene time. Black tadpole symbols denote bedding. A) Eocene fluvial-lacustrine strata (Arkose Ridge Formation) deformed by asymmetric folds and reverse faults in the southern Talkeetna Mountains in the remnant Matanuska basin (MB on Fig. 1). Height of outcrop is N150 m. B) Oligocene fluvial-lacustrine strata (Oc) exposed in a footwall syncline adjacent to a reverse fault along the south flank of the central Alaska Range in the remnant Colorado Creek basin (CB on Fig. 1). Jurassic–Cretaceous marine strata (KJk) are exposed in the hanging wall of the reverse fault. Height of outcrop with Oligocene strata is N 200 m. Snow covered peak in upper left side of photo is Mt. McKinley (elevation 20, 320 ft.); the Yakutat slab is located ~ 150 km beneath this mountain based on geophysical imaging.

pathway predominantly drained source areas exposed north and northeast of the basin, including the central Alaska Range (Fig. 1). Another key pathway derived sediment from eastern and northeastern

source regions, including the Talkeetna Mountains (Fig. 1). We infer that exhumation of the flat-slab region resulted in relative subsidence in the Cook Inlet basin, created prolific sediment sources to the east and

Fig. 7. Late Eocene–Quaternary stratigraphy of sedimentary basins in south-central Alaska (1—Flores et al., 2004; 2–4—Trop and Ridgway, 2007; 5—Richter et al., 1990 and Trop et al., 2007). Note lack of sedimentary strata deposited after Oligocene time in the region above flat-slab subduction and increase in thicknesses of post-Oligocene sedimentary strata in basins located along western and northern perimeter of flat-slab subduction. Also note northwestward younging of volcanism and sedimentation in the Wrangell volcanic belt derived from abundant geochronologic and palynological data (black vertical lines). Quaternary strata in the Cook Inlet and Tanana basins are unnamed. Time scale of Walker and Geissman (2009).

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Fig. 8. Photos of Neogene sedimentary strata that record enhanced sediment accumulation along the perimeter of the flat-slab region. A) and B) Strata deposited in the Cook Inlet basin located along the western perimeter of the flat-slab region (CB on Fig. 1). A) Miocene fluvial sandstone, mudstone, and coal (Beluga Formation). Person circled for scale. B) Pliocene fluvial sandstone and mudstone (Sterling Formation). Note the thick channel complex between the white dashed lines. Arrow points to person for scale. C), D), and E) Strata deposited in the Tanana transpressional foreland basin located along the northern margin of the flat-slab region (TB on Fig. 1). C) Miocene coal, sandstone, and mudstone (Usibelli Group) deposited in fluvial channels and floodplain mires. Coal beds are up to 20 m thick. Spruce forest at bottom of photo for scale. D) Miocene lacustrine mudstone with seasonal varves (Usibelli Group) deposited in floodplain lakes. Person (lower right) for scale. E) Pliocene conglomerate and sandstone (Nenana Gravel) deposited in alluvial fans. Person (lower right) for scale. F) Miocene strata deposited in transtensional basins along the eastern margin of the flat-slab region in the Wrangell volcanic belt (WVB on Fig. 1). Miocene alluvial-lacustrine conglomerate, mudstone, tuff, and lava flows (Mf-Frederika Formation) are overlain by lava flows and volcanic breccia (Mw-Wrangell Lava). Height of outcrop is N 700 m. Black tadpole symbols denote bedding. G) Mount Wrangell, a 4317-m (14,163 ft)-high andesite shield volcano in the Wrangell Mountains with documented historical activity. View is to the northeast. Photograph by B. Cella, U.S. National Park Service, 1987.

northeast of the basin, and led to southwestward progradation of a large clastic wedge away from the region presently above flat-slab subduction and into the basin (Finzel, 2010; Finzel et al., 2007). An interpreted northern extension of the Cook Inlet basin, the Susitna basin (Fig. 1; Haeussler, 1998; Haeussler et al., 2002), also appears to contain a thick section of Cenozoic strata. Very little is known about the age and stratigraphy of the basin, but based on three drill holes and gravity data there may be as much as 4 km of Paleocene–Miocene nonmarine clastic strata (Merritt, 1986; Meyer and Boggess, 2003). Much more data is needed, but we tentatively interpret the Paleocene–Miocene strata of the Susitna basin to be part of the large clastic wedge along the western perimeter of flat-slab subduction.

3.2. Northern perimeter The Tanana basin is positioned directly north of the flat-slab region (Fig. 1). Along the southern margin of the basin, Cenozoic strata are 2–3 km thick and have been exhumed by thrust faults that form the foothills along the north side of the Alaska Range (Ridgway et al., 2002, 2007). Recent studies clearly show that the foothills are part of an active northward-propagating thrust belt that is deforming Neogene strata of the Tanana foreland basin (Bemis and Wallace, 2007; Lesh and Ridgway, 2007). The Usibelli Group consists of 970 m of mainly middle and upper Miocene strata that were deposited in fluvial, lacustrine, and peat bog environments (Fig. 7). In one part of the Tanana basin, the Rex Creek area, the oldest part of the Usibelli

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at least Late Eocene time and continues to the present (e.g., EberhartPhilips et al., 2006; Ridgway et al., 1995, 2002; Ruppert et al., 2008; Trop et al., 2004). 3.3. Eastern perimeter

Fig. 9. Maximum stratigraphic thickness (Flores et al., 2004) versus time plot for Paleocene to Pliocene strata in Cook Inlet basin. Note that accumulation rates increased significantly during Oligocene time and continued to increase through Pliocene time. All formation ages are constrained by biostratigraphy, including palynology and leaf macrofossils (Kirschner and Lyon, 1973; Magoon et al., 1976; Wolfe and Tanai, 1980), with additional age control in the Beluga and Sterling formations derived from geochronology of interbedded tuffs (Dallegge and Layer, 2004; Reinink-Smith, 1990; Triplehorn et al., 1977; Turner et al., 1980). Time scale of Walker and Geissman (2009).

Group may extend into Late Eocene time (Wolfe and Tanai, 1980), but at the type section and in most studied parts of the basin, the group is entirely Miocene in age (Leopold and Liu, 1994; Ridgway et al., 2007). Thick successions of lacustrine mudstone and coal seams N20-m-thick imply regional basin subsidence during sediment accumulation (Fig. 8C–D). The Nenana Gravel consists of 1200 m of Pliocene strata that were deposited in northward-prograding alluvial-fan and braidplain environments that record erosion of high topography in the central Alaska Range (Fig. 8E). The available stratigraphic data suggest that the Tanana basin formed in flexural response to northward propagation of shortening along the southern margin of the basin (Ridgway et al., 2007). We interpret the Tanana basin as a transpressional foreland basin because its development has been coeval with dextral strike-slip displacement on the Denali fault system. Cenozoic displacement on the Denali fault system started by

3.3.1. Wrangell volcanic belt The eastern perimeter of flat-slab subduction is bounded by the Wrangell volcanic belt (WVB on Fig. 1) and active strike-slip faults. The WVB consists of lava flows, lava domes, pyroclastic strata, and nonmarine sedimentary strata with maximum preserved thicknesses N3000 m (Fig. 8F–G; Richter et al., 1990; Skulski et al., 1992). Strata crop out near regional northwest-striking strike-slip faults (Denali, Duke River, and Totschunda faults on Fig. 1) and are locally cut by northstriking normal faults (Eisbacher and Hopkins, 1977; MacKevett, 1978; Richter et al., 2006). Extensive geochronologic and palynologic data document northwestward younging of WVB strata (Fig. 7; Richter et al., 1990; Ridgway et al., 1995; Skulski et al., 1992). The southeastern WVB consists of Eocene–Upper Miocene alluvial-lacustrine sedimentary strata and eruptive centers with mainly transitional and minor alkaline and calc-alkaline geochemical compositions (Cole and Ridgway, 1993; Ridgway and DeCelles, 1993; Skulski et al., 1991). The northwestern WVB is characterized by Middle Miocene–Holocene alluvial-lacustrine sedimentary strata and lavas with transitional to calc-alkaline geochemical compositions typical of subduction-related volcanic suites (Preece and Hart, 2004; Trop et al., 2007). We interpret the WVB stratigraphic record as the product of transtensional tectonics and partial melting along the eastern edge of the shallow Yakutat slab. We suggest that northwestward insertion of a shallow slab beneath Alaska prompted northwestward-migrating eruptive centers and narrow (b20 km wide) transtensional basins along northwest-striking strike-slip faults (Totshunda and Duke River faults on Fig. 1). Stratigraphic evidence for lacustrine deposition and coal formation indicate active subsidence within basins during deposition. The orientation of north-striking normal faults is consistent with east–west extensional subsidence related to dextral displacement along adjacent strike-slip faults (Trop et al., 2007). The lack of geophysical evidence for a present-day subducted slab deeper

Fig. 10. Age probability plot showing the zircon age distributions for seven sandstone samples from middle Eocene–Pliocene strata in Cook Inlet basin and age ranges for potential source terranes in southern Alaska. Histograms are in 10 m.y. intervals. A total of 634 grains were analyzed via laser ablation ICP mass spectrometry at the University of Arizona LaserChron Center. All b 250 Ma ages are plotted (n = 590). See Finzel (2010) for methods and raw age data. Inset is a plot of U/Th versus age for all 590 grains. Note that ~ 98% of the grains have U/Th values N10. See Fig. 1 for sample locations and general geographic locations of source terranes described below. Potential source terranes located west of the flatslab region include the Western Alaska Range (WAR on Fig. 1), where the Aleutian Arc (AA; 0–10 Ma) and the Western Alaska Range igneous belt (WAR; 30–45 Ma) crop out. Potential igneous source terranes located above and east of the flat-slab region include the Caribou Creek-Central Talkeetna Mountains volcanics (CTM; 35–60 Ma), the Central Alaska Range belt (CAR; 70–120 Ma, CAR on Fig. 1), and the Chisana (CS; 105–140 Ma) and Chitina (CT; 135–175 Ma) arcs (both located between the St. Elias Mountains and the Denali fault on Fig. 1). The Alaska Range–Talkeetna Mountains batholith (ATM; 55–74 Ma) and the Talkeetna Arc (TA; 153–201 Ma) occur in both areas, spanning the northwest side of Cook Inlet to the Talkeetna Mountains. References for source terrane age data are discussed in Section 3.1.

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than 50 km beneath the WVB (Figs. 2 and 3A; Eberhart-Philips et al., 2006; Qi et al., 2007) together with the presence of adakite compositions in volcanic rocks from Mounts Drum and Churchill in the WVB (Fig. 1; Preece and Hart, 2004) indicate partial melting along the northeastern edge of the Yakutat slab. Adakite melts commonly form by slab melting when the downgoing slab is young (b20–30 Ma) and at shallow depths (b85 km), the dip of the downgoing plate is very shallow, or the plate margin is characterized by elevated shear stresses, highly oblique subduction, or slow subduction (Drummond et al., 1996; Gutscher et al., 2000; Yogodzinski et al., 1995, 2001). Spatial variations in WVB geochemical compositions imply magmatism mainly along extensional leaky strike-slip faults in the southeast (Skulski et al., 1991, 1992) and increased flux of slab-derived constituents into the mantle source in the northwest (Preece and Hart, 2004), consistent with northwestward-directed subduction of the flat slab along the curved continental margin of eastern Alaska. 3.3.2. Copper River basin The Copper River basin lies between the Talkeetna Mountains and the WVB along the eastern perimeter of the flat-slab region (Fig. 1). Exploratory wells encountered 580 m of Cenozoic strata and 1715 m of Upper Jurassic–Upper Cretaceous strata (Alaska Geological Society, 1970a,b). Subsurface data, including seismic profiles and aeromagnetic and gravity surveys, indicate a maximum thickness of 1200 m for the Cenozoic strata and 7000 m for the Mesozoic strata (Fuis and Plafker, 1989; Meyer and Boggess, 2003; Meyer and Saltus, 1995). The specific ages of the Cenozoic strata in this basin are poorly known making it difficult to determine any possible relationship between timing of deposition and possible flat-slab processes. 4. Discussion 4.1. Upper plate record of flat-slab subduction in Alaska We propose that a flat subduction style initiated in southern Alaska ca. 20 m.y. earlier than previously inferred and that the Yakutat microplate has been subducting at a shallow angle since the initiation of its subduction beneath Alaska in late Eocene–early Oligocene time. This premise is supported by recently published geophysical data that document thick oceanic crust for the entire Yakutat slab, together with our integrated geochronologic, thermochronologic, stratigraphic, and provenance data from the region presently above and around flat-slab subduction in southern Alaska. Our data provide constraints on the timing of subduction-related magmatism, enhanced rock uplift and erosion, and development of sedimentary basins. In the flat-slab region, exhumation initiated near the southern margin ca. 43 Ma (Eocene) and continues through Neogene time throughout the entire flat-slab region, magmatism ceased at ca. 32 Ma (Oligocene), and deposition in sedimentary basins ended by ca. 23 Ma (Miocene). Sedimentary basins positioned along the western and northern perimeter of the flat-slab region record enhanced sediment accumulation rates and sediment delivery from bedrock sources exhumed above the flat-slab region beginning in late Oligocene and middle Miocene time respectively. Subduction-related volcanism and basin development along strike-slip faults that bound the eastern perimeter of the flat-slab region initiated ca. 26 Ma (Oligocene) and migrated northwestward during Miocene–Holocene time. Collectively, we interpret these age constraints, which were derived from different proxies, to represent late Eocene–early Oligocene initiation and continued shallow subduction of the Yakutat microplate. All these lines of evidence are consistent with flat-slab processes being important from Oligocene to present time along the southern margin of Alaska. An alternative hypothesis to flat-slab subduction of the Yakutat microplate beginning during Oligocene time is that shallow subduction may have been a lingering product of subduction of a spreading

ridge beneath southern Alaska ca. 61–50 Ma. Oriented subparallel to the paleo-trench, the spreading ridge subducted obliquely from west to east beneath southern Alaska judging by the diachronous ages of near-trench plutons related to slab-window magmatism (e.g., Haeussler et al., 2003). Adjacent to the spreading ridge, there would have been very young oceanic crust subducting at the trench along the southern margin of Alaska that presumably would have been relatively warm and buoyant. An argument against the ridge subduction hypothesis comes from our compilation of geochronologic data that indicates that subduction-related magmatism resumed in southern Alaska after the spreading ridge passed sometime after 50 Ma (Fig. 4). Therefore, whatever the age of the crust subducting beneath southern Alaska between ca. 50 and 43 Ma, it was subducting at a steep enough angle to produce a volcanic arc. The presence of arc magmatism after early Paleogene ridge subduction suggests that ridge subduction is a less likely explanation for the upper plate processes documented in our databases. 4.2. A larger Yakutat microplate? Comparison of geochronologic, thermochronologic, and stratigraphic data from above and to the west of the present-day flat-slab region allows for a reevaluation of models on the kinematic history of the Yakutat microplate. Here, we hypothesize that the original leading edge of the Yakutat microplate may have been wider and extended farther west along strike compared to the current geometry. Similar to the present-day flat-slab region, published reconnaissance thermochronologic data from areas to the west also record a possible general northward younging of the onset of rock exhumation from the Kenai Peninsula towards the western Alaska Range northwest of Cook Inlet (Fig. 1). In the Chugach Mountains on the Kenai Peninsula, apatite (U-Th)/He ages record exhumation of the accretionary prism between ca. 40 and 12 Ma (8 on Fig. 1; Buscher et al., 2008). Farther inboard (~200 km), apatite FT ages from Paleogene granite in the western Alaska Range record rapid exhumation at ca. 23 and 6 Ma, with one apparently anomalous sample recording rapid cooling at ca. 35 Ma (9 on Fig. 1; Haeussler et al., 2008). Geochronologic data from igneous rocks indicate that subduction-related magmatism ceased in the western Alaska Range ca. 30–23 Ma (Fig. 4; Moll-Stalcup, 1994) and did not resume on the Alaska Peninsula until ca. 10 Ma (Reinink-Smith, 1990). Detrital data from Middle Miocene–Pliocene strata in the forearc basin also demonstrate a lack of Early–Middle Miocene aged igneous sources (Fig. 10). Farther to the west, Aleutian arc magmatism synchronously waned ca. 30 Ma and resumed ca. 16–11 Ma (Jicha et al., 2006). If the same tectonic process is responsible for upper plate processes above and inboard of the present-day flat-slab region as well as for these areas to the west, namely flat-slab subduction, then it is possible that shallow subduction of the Yakutat microplate impacted a larger extent of southern Alaska in the past than what is evident today. An alternative travel history for the Yakutat microplate has been previously proposed based on plate reconstructions and seismic imaging of accretionary prism strata near the Kenai Peninsula (Fruehn et al., 1999). In that model, the Yakutat microplate is assumed to be coupled with the Pacific plate since at least Pliocene time. Using published plate motion models, these authors propose that extraction of the Yakutat microplate from beneath Alaska would cause its southwestern trailing edge, coincident with the Transition fault, to pass beneath the accretionary prism outboard of the Kenai Peninsula from ~ 3.5 to 2 Ma. Evidence for passage of a buoyant Yakutat slab through this region includes truncation and erosion of pre-Neogene accretionary prism strata and expansion of the Neogene accretionary prism southward away from the present-day southwestern boundary of the Yakutat microplate (Fig. 1). This model differs from the Plafker (1987) model in that the original western extent of North America– Yakutat interaction would have extended much farther west, at least

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to the southern end of the Kenai Peninsula, the flat Yakutat slab would have potentially passed beneath much of the Cook Inlet forearc basin during middle to late Cenozoic time, and the Yakutat microplate would not necessarily have been located adjacent to the western North American margin while it traveled north toward Alaska. Recent studies support the proposal by Plafker (1987) that the southeastern unsubducted portion of the Yakutat microplate was positioned along the western continental margin of North America since Eocene time. For example, stratigraphic and detrital provenance studies from Eocene and younger sedimentary strata exposed on the microplate document deltaic-shallow marine deposystems that were sourced from continental margin source terranes in southeastern Alaska or western Canada (Landis, 2007; Perry et al., 2009). Similarly, age data and geochemical compositions from igneous rocks in both the microplate and adjacent portions of Alaska are attributable to slab-window magmatism associated with subduction of a spreading ridge beneath the continental margin during Eocene time, requiring juxtaposition of the microplate with southeastern Alaska or western Canada (Bradley et al., 2003; Davis and Plafker, 1986; Haeussler et al., 2003). We suggest a scenario that accounts for uplift and truncation of the accretionary prism strata in the Kenai Peninsula region while keeping the eastern, unsubducted portion of the slab adjacent to the North American continental margin. We postulate that the original width of the subducting slab may have been greater than the width presently indicated by the unsubducted portion or the geophysically imaged subducted slab. Published geophysical images have only loosely constrained the nature of the boundary between the Pacific and Yakutat plates at shallow depths (Eberhart-Philips et al., 2006). Thus, the subducted portion of the slab could have been wider and therefore interacted with a broader extent of the southern Alaska margin in the past. Alternatively, the Yakutat slab may not have been wider but instead stresses transmitted inboard from the flat-slab region may have contributed to the deformation to the west. The eastward decrease in the width of the accretionary prism towards the presentday flat-slab region (Fig. 1), however, suggests that whatever perturbed the depositional system outside the present-day flat-slab region traveled from west to east along the southern margin of Alaska. Presumably, as the disturbance migrated to the northeast, sedimentation resumed in the accretionary prism, leading to the present-day geometry of the prism as described above. In summary, the possibility exists that the original width of the Yakutat microplate may have been wider than its presently imaged dimensions, but much more data is needed to test this hypothesis. 4.3. Comparison with other modern flat-slab margins The widely distributed, diffuse deformation and the transpressional tectonic regime documented in southern Alaska are common characteristics of regions undergoing oblique subduction of shallow slabs. Insertion of a shallow slab into a subduction margin commonly causes regional shortening and exhumation of the upper plate as a result of the inboard propagation of plate boundary compressive stresses and isostatic adjustments (Espurt et al., 2007, 2008; Gutscher, 2002). Earthquake focal mechanisms demonstrate that transpressive stress regimes dominate many modern flat-slab margins, including southern Alaska, New Guinea, northern Columbia, Japan, and segments of the Andean margin (Gutscher et al., 2000; Haeussler, 2008). The transpressive nature and broad distribution of deformation several hundred kilometers inboard from the plate margin in these regions is attributable to coupling of the subducted flat slab to the upper plate and oblique convergence between the slab and the upper plate. This is consistent with enhanced contact between the two plates and the cooler temperature due to lack of an asthenospheric wedge (e.g., Gutscher, 2002). Transpressive shortening of the upper plate prompts regional exhumation, a process that may be accom-

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modated by discrete pre-existing structures or zones of weakness (Espurt et al., 2007, 2008; Gutscher, 2002). Strong interplate coupling in regions experiencing both flat-slab subduction and oblique convergence may also lead to strain partitioning far inboard of the subduction zone (Chemenda et al., 2000; Pabellier and Cobbold, 1996; Pinet and Cobbold, 1992). For example, along the northwestern Andean margin and in southwestern Japan, major strike-slip faults occur ~300–400 km inboard of the trench. In southern Alaska, the Castle Mountain fault accommodates ~ 3 mm/yr of dextral-oblique strike-slip displacement ~250 km inboard of the trench (Willis et al., 2007). Even farther inboard, the Denali fault accommodates ~10 mm/yr of dextral-oblique strike-slip motion ~400 km inboard of the plate margin (Fig. 1; Haeussler, 2008). Flat-slab subduction occurs when relatively buoyant oceanic crust enters a subduction zone and causes the slab to progressively flatten (Espurt et al., 2008; Gutscher et al., 2000). Buoyant oceanic crust results from two main characteristics: a moderate to young age of the subducting crust (b50 m.y.) in combination with crust thicker than ~15 km. Thick oceanic crust often characterizes aseismic ridges and oceanic plateaus. Two well-documented modern examples of thick oceanic crust subducting beneath continental margins are the aseismic Nazca Ridge in South America and the Ontong Java Plateau in the south Pacific (Cowley et al., 2004; Espurt et al., 2007, 2008; Hampel, 2002; Knesel et al., 2008; Mann and Taira, 2004; Phinney et al., 2004; Taira et al., 2004; vonHuene et al., 1999). In these regions, as well as in southern Alaska, flat-slab subduction of thick oceanic crust has resulted in upper plate shortening and exhumation above the flattened slab, cessation of magmatism within a few million years of the onset of shallow subduction, and erosion and subsequent reestablishment of the accretionary prism during and after lateral migration of the shallow slab. The trench parallel widths and crustal thicknesses of the Nazca Ridge and Yakutat microplate are comparable, and although the Nazca Ridge has been subducting for only ca. 11–12 Ma compared to ca. 35 Ma for the Yakutat microplate inferred from this study, a higher convergence rate in South America has led to similar amounts of subducted material in the two regions. Both margins are characterized by oblique convergence, which, in South America, has resulted in the southward migration of the Nazca Ridge along the South American trench (Hampel, 2002). In Alaska and South America, exhumation and deformation in the forearc region was coincident with the arrival of thick crust at the trench, magmatism ceased above the subducting flat slab ca. 3–8 m.y. later, strong interplate coupling led to deformation within the upper plate that extended hundreds of kilometers inboard, and accretionary prism sedimentation resumed along strike after the flat slab migrated laterally (Espurt et al., 2008; Hampel, 2002; vonHuene et al., 1999). In South America, stratigraphic data record uplift of forearc depocenters coincident with passage of the Nazca Ridge. The Ontong Java Plateau is the world's largest oceanic plateau, covering about 1.9 million km2, with an average crustal thickness of ~30–35 km (Knesel et al., 2008; Mann and Taira, 2004). Approximately 1000 km of the trench parallel width of the plateau is currently subducting along the North Solomon trench. The Ontong Java Plateau has only been subducting for ca. 4–6 Ma, resulting in a total subducted slab length of ~ 200–300 km (Cowley et al., 2004; Mann and Taira, 2004; Phinney et al., 2004; Taira et al., 2004). Prior to arrival of the plateau at the subduction zone, Miocene and Pliocene arc volcanism was prevalent in the Solomon Islands region. After the plateau arrived at the trench ca. 4–6 Ma, arc magmatism began to wane and is less common today than it was prior to Pliocene time, major folding and uplift occurred in the accretionary prism, and intra-arc normal faults were reactivated as thrust faults. Presently, convergence between the Pacific plate and Australian plate is accommodated at both the North Solomon trench and a young northeast-dipping subduction zone that has developed in the backarc region.

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Several important characteristics are shared among the Alaska, South American, and south Pacific flat-slab settings. First, shallow subduction of buoyant oceanic crust led to increased plate coupling, which resulted in upper plate shortening, rock uplift and exhumation above the flat-slab region. Second, subduction-related magmatism waned or ceased b10 m.y. after the onset of shallow subduction. Third, shoaling or inversion of basins above the flat-slab region led to an increase in coarser sedimentation and/or a lack of deposition. Fourth, the accretionary prism was deformed or eroded above the flat slab, and in the case of Alaska and South America, sedimentation within the prism was reestablished after the shallow slab passed laterally along the margin. The most distinct difference between Alaska and these other regions is that the northeastern boundary of the Yakutat microplate is a strike-slip margin. The effect on the upper plate of this boundary has been the propagation of transtensional tectonics along the northeastern edge of the subducting slab. This is reflected in the northwestward younging of slab-edge volcanism and basin development along the Denali–Duke River strike-slip fault system in the WVB (Figs. 1 and 7). 5. Conclusions Integration of thermochronologic, geochronologic, stratigraphic, and provenance data from the upper plate area above and around flatslab subduction of the Yakutat microplate in southern Alaska provides evidence that flat-slab processes have been occurring since late Eocene–early Oligocene time. Above the flat slab, regional exhumation initiated ca. 43 Ma, magmatism ceased ca. 32 Ma, and deposition in sedimentary basins ended ca. 23 Ma. Along the western and northern perimeter of the flat-slab region, sedimentary basins experienced enhanced subsidence and sediment delivery from the flat-slab region beginning in late Oligocene and middle Miocene time, respectively. In the Wrangell volcanic belt along the eastern margin of the flat-slab region, volcanism and basin development along dextral strike-slip faults initiated ca. 26 Ma (Oligocene) and migrated northwestward during Miocene–Holocene time. We propose that these data support shallow subduction of the buoyant Yakutat microplate starting in late Eocene–early Oligocene time. Furthermore, stratigraphic and thermochronologic data from the Kenai Peninsula and western Alaska Range, west of the present-day flat-slab region, indicate flat-slab processes may have been active in those regions previously. Consequently, we postulate that the subducted portion of the Yakutat microplate may have extended farther west along strike and affected a larger region of southern Alaska than the width presently indicated by the unsubducted portion of the Yakutat microplate or the geophysically imaged subducted slab. Modern convergent margins influenced by flat subduction styles are shaped by processes similar to those observed in southern Alaska, including regional exhumation, cessation of magmatism, shoaling or inversion of sedimentary basins, and deformation or erosion of the accretionary prism above the flat-slab region. Supplementary materials related to this article can be found online at doi:10.1016/j.epsl.2011.01.014. Acknowledgements Primary funding was provided by the National Science Foundation, Donors of the Petroleum Research Fund administered by the American Chemical Society, and the Bucknell University Program for Undergraduate Research. Reviewers S. Roeske, P. Haeussler, and R. Dorsey offered constructive reviews that helped us improve the manuscript. We thank A. Till, W. Wallace, and B. McNulty for reviews of an earlier version of the manuscript. We also thank Bucknell University undergraduate students E. Bauer, R. Delaney, C. Kortyna, R. Tidmore, and J. Witmer for field assistance and thesis research.

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