Influence of the Amlia Fracture Zone on the Evolution of the Aleutian

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Inﬂ uence of the Amlia fracture zone on the evolution of the Aleutian Terrace forearc basin, central Aleutian subduction zone

H.F. Ryan1,*, Amy E. Draut2, Katie Keranen3, and David W. Scholl1 1U.S. Geological Survey, 345 Middleﬁ eld Road, Menlo Park, California 94025, USA 2U.S. Geological Survey, 400 Natural Bridges Drive, Santa Cruz, California 95060, USA 3University of Oklahoma, School of Geology and Geophysics, 100 E. Boyd Street, Norman, Oklahoma 73019, USA

ABSTRACT large mass failures of the summit platform, tsunamis. This section of the Alaska-Aleutian as evidenced by the presence of thick mass subduction zone has produced great, tsunami- During Pliocene to Quaternary time, the transport deposits of primarily Quaternary genic earthquakes in the recent past, including central Aleutian forearc basin evolved in age that are found in the forearc basin west the Mw 8.6 event in 1957, which caused signifi - response to a combination of tectonic and cli- of the Amlia fracture zone. cant damage to structures in Hawaii from tsu- matic factors. Initially, along-trench trans- nami waves as high as 16 m (Lander and Lock- port of sediment and accretion of a frontal INTRODUCTION ridge, 1989). prism created the accommodation space to The central Aleutian subduction zone is an allow forearc basin deposition. Transport of Sedimentary basins around island arcs obliquely subducting margin where large struc- suffi cient sediment to overtop the bathymet- provide some of the most complete records tures on the downgoing plate propagate along rically high Amlia fracture zone and reach of active-margin tectonic evolution. Forearc the arc, affecting the forearc in a time-trans- the central Aleutian arc began with glacia- basins, in particular, form structurally and fi ll gressive manner. The main structure we focus tion of continental Alaska in the Pliocene. with sediment in response to a complex inter- on is the impact of the westward- propagating As the obliquely subducting Amlia fracture play between tectonic controls and sediment Amlia fracture zone on the evolution of the zone swept along the central Aleutian arc, sources (e.g., Dickinson, 1974, 1995; Marsaglia central Aleutian forearc basin and the associ- it further affected the structural evolution and Ingersoll, 1992; Underwood et al., 1995). ated amount of coupling along the plate inter- of the forearc basins. The subduction of the In addition to providing a record of the tectonic face. The forearc basin response to fracture Amlia fracture zone resulted in basin inver- history of a convergent margin, the location of zone propagation is documented in differences sion and loss of accommodation space east forearc basin depocenters may record the loca- in style of deformation and the location and of the migrating fracture zone. Conversely, tions of areas of large slip along the megathrust migration of basin depocenters. This study west of Amlia fracture zone, accommodation during great subduction zone earthquakes (e.g., examines the sedimentary architecture in detail space increased arcward of a large outer-arc Wells et al., 2003; Song and Simons, 2003; over a broad area of the central Aleutian forearc high that formed, in part, by a thickening Fuller et al., 2006; Llenos and McGuire, 2007; basin, incorporating age control based on Deep of arc basement. This difference in defor- Rosenau and Oncken, 2009). Since forearc Sea Drilling Project (DSDP) cores recovered at mation is interpreted to be the result of a basins are long-lived features, they serve as a sites 186 and 187. We link forearc stratigraphy variation in interplate coupling across the record of subsidence above the plate interface to (1) the westward propagation of the Amlia Amlia fracture zone that was facilitated by over time scales on the order of a million years. fracture zone through time, (2) the importance increasing subduction obliquity, a change in We investigated what forearc basin evolution of strike-slip faulting in the forearc as subduc- orientation of the subducting Amlia fracture can tell us about the relative importance of vari- tion becomes more oblique, (3) the relatively zone, and late Quaternary intensifi cation of ous subduction-zone characteristics in causing rapid evolution of forearc structures during the glaciation. The change in coupling is mani- great earthquakes, which remains challenging to last 1 m.y., including the infl uence of climate fested by a possible tear in the subducting assess, given the relative rarity of such events. on trench sedimentation and plate coupling. slab along the Amlia fracture zone. Differ- Here, we analyzed the Pliocene and Quaternary Finally, by comparing evidence for processes ences in coupling across the Amlia fracture evolution of forearc basin depocenters along that affect the along- and across-strike struc- zone have important implications for the the central Aleutian convergent margin. The tural and stratigraphic character of the forearc, location of maximum slip during future Alaska-Aleutian subduction zone is one of the we evaluate the potential for the central Aleu- great earthquakes. In addition, shaking longest in the world and is capable of produc- tian forearc region to produce tsunamis trig- during a great earthquake could trigger ing great earthquakes that generate transoceanic gered by great earthquakes.

*Corresponding author: [email protected].

Geosphere; December 2012; v. 8; no. 6; p. 1254–1273; doi:10.1130/GES00815.1; 15 ﬁ gures. Received 30 April 2012 ♦ Revision received 20 June 2012 ♦ Accepted 3 October 2012 ♦ Published online 16 November 2012

GEOLOGIC AND TECTONIC SETTING of the Oligocene to late Miocene middle series lowing enhanced glacial erosion in southeastern is thought to have accompanied a phase of sub- Alaska after the Yakutat–North America col- The present confi guration of the central stantially reduced magmatism, and erosion and lision began (Scholl et al., 1987; Gulick et al., Aleutian forearc is the result of tectonic evolu- subsidence of the arc platform (Scholl et al., 2007). An outer-arc high formed as the result of tion dating at least to early Eocene time. The 1983, 1987). The middle and lower series under- accretion and underplating of sediment beneath Aleutian Ridge is subdivided generally into lie the modern forearc basin and do not exhibit the arc massif along a primary splay fault. This three chronostratigraphic units known as the basin-fi lling sedimentation, which indicates that created the accommodation space for the depo- lower, middle, and upper series (Scholl et al., the Aleutian arc in this region did not include sition of sediment in the forearc basin. Upper- 1983, 1987), which refl ect three major phases of a true forearc basin depression until after late series deposits fi ll the forearc basin depocenters, tectonic evolution of the arc since the Eocene, Miocene time (Scholl et al., 1983, 1987; Har- resting unconformably on or faulted against ca. 50 Ma. The lower series formed during an bert et al., 1986). At ca. 5–6 Ma, an accretionary middle- and lower-series rocks; thus, the forearc initial, constructive phase of voluminous arc frontal prism formed as the result of signifi cant basin is underlain entirely by arc massif. Forearc magmatism that accompanied subduction of the sediment underthrusting, likely triggered by an basin deposits are composed of volcaniclastic Kula plate (Scholl et al., 1983, 1987). Formation increase in sediment supply to the trench fol- and pelagic sediment (e.g., Scholl et al., 1987). N °

Bering 3 Sea 53°N 5

Pacific Ocean

AmliaAmlia basinbasin SeguamSeguameguam AmuktaAmukta AtkaAtka TaTTanagaanagaga AmliaAmlia AAdakdakdak AleutianAleutAleutianan RidgeRRiidge 52°N

Aleutian Terrace

2E994) 3 FIG. 14 7L9 AB

8 Adak 1 (12 9 Canyon HR DSDP 186 FIG.

8 (13L980)

HRSZ 51°N FIG.

)

FIG. 7 (12L981) 7 FIG. (5L981

FIG. 4 (10L981) 6

Aleutian Trench 72 mm/yr FIG. FIG.

FIG. 5 (6L981) 50km AFZ

178°W 176°W 174°W 172°W 50°N

Figure 1. Location map showing study area in the Andreanof Islands, which stretch from Tanaga to Amukta Islands. The entire Aleutian arc is shown in the inset, including marine magnetic anomalies from Atwater (1989). Topography and bathymetry for both maps are from Lim et al. (2009). The outlines of Atka Basin (AB) and Hawley Ridge (HR) are dashed in white. Multichannel seismic-refl ection (MCS) track lines are shown by yellow lines; those displayed as fi gures are labeled. Locations of heat-fl ow sites are shown by stars (GeoPRISMS Data Portal, 2011). White dots on Aleutian Ridge delineate the subsided southern edge of the summit platform. Hawley Ridge shear zone (HRSZ) is shown by red line. The Andreanof block (modifi ed from Geist et al., 1988) is shown by thin white line and stippled pattern. AFZ—Amlia fracture zone; DSDP—Deep Sea Drilling Project.

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Outer forearc basin sediment was sampled at 1; LaForge and Engdahl, 1979; Geist et al., A major structure on the Pacifi c plate under- DSDP site 186 and consisted predominately of 1988). Geist et al. (1988) used geomorphic and thrusting the central Aleutian Ridge is the diatomaceous silty clay and ooze with interbed- structural elements of the arc massif imaged Amlia fracture zone. The Amlia fracture zone ded volcanic ash and pumice, as well as lami- on seismic-refl ection profi les, supported by is an ~1-km-high, 20- to 40-km-wide bathy- nated, volcaniclastic sand-and-silt turbidites as paleomagnetic data from Amlia Island (Har- metric feature now situated near longitude thick as >4 m (Scholl and Creager, 1973; Stew- bert, 1987), to show that the eastern sector 173°W (Fig. 1). Owing to oblique subduction, art, 1978). The volcaniclastic deposits evidently of the Andreanof block has rotated 15°–25° the Amlia fracture zone is currently migrating were derived largely from the Aleutian arc, to clockwise about a vertical axis, with associated westward along the margin at a rate of over the north, delivered from the arc front via sub- strike-slip offset in the forearc and formation of 2 cm/yr (DeMets, et al., 2010). It juxtaposes marine canyons that debouch into the forearc pull-apart basins along the arc summit. Seafl oor 57 m.y. oceanic crust on its west side against basin (Underwood, 1986). imagery and seismic-refl ection profi les indicate 65 m.y. crust to the east; magnetic lineations are In the region of the Andreanof Islands, the that oblique convergence also causes right-lat- offset 220 km in a left-lateral sense across the oceanic crust of the Pacifi c plate obliquely eral strike-slip offset along the Hawley Ridge fracture zone. The older seafl oor on the Pacifi c underthrusts (30° to normal) the Aleutian shear zone, which forms the southern margin plate east of the fracture zone is several hundred island arc (DeMets et al., 2010). In order to of the Andreanof block (Ryan and Scholl, meters deeper than that to the west. In addition, accommodate shear stress along the subduc- 1989). Along the northern edge of the Andre- the younger seafl oor west of the Amlia fracture tion zone caused by oblique convergence, anof block, a complex shear zone accommo- zone exhibits a more complex bathymetry, with the Aleutian Ridge has separated into blocks dates slip partitioning within the volcanic arc the presence of more small seamounts on the that rotate clockwise and translate to the west (Ekstrom and Engdahl, 1989; Lallemant and subducting plate (Fig. 2; Lonsdale, 1988). The (Spence, 1977; Geist et al., 1988). The Andre- Oldow, 2000). The shallow upper plate at the Amlia fracture zone forms a barrier that effec- anof block is defi ned geomorphically, seismi- eastern end of the Andreanof block is deformed tively reduces the volume of westward sedi- cally, and paleomagnetically as extending from by transverse left-lateral strike-slip and normal ment transport by turbidity currents along the Adak Canyon to east of Seguam Island (Fig. faulting (Ruppert et al., 2012). trench, leading to greater thickness of trench

5025 0 50 km AMUKTA ATKA AMLIA TANAGA SEGUAM

ADAK 51°N

AFZAFZ 49°N

177°W177°W 175°W175°W 173°W173°W 171°W171°W

Figure 2. GLORIA (Geological LOng-Range Inclined Asdic) sidescan sonar imagery (Paskevich et al., 2010) showing high backscat- ter reﬂ ecting relief on the Paciﬁ c plate. West of Amlia fracture zone (AFZ), the subduction plate has more relief than to the east. The white line shows location of arcward edge of the trench.

1256 Geosphere, December 2012

sediment, and corresponding greater fl ux of occur within the United States during historic (GPS) measurements to show that there is strong sediment-derived melt in arc magmatism, east times (Johnson et al., 1994). Unfortunately, the coupling between the upper and lower plates in of the fracture zone compared to west of it source parameters of the 1957 event are not well the western part of the Andreanof block and little (Scholl et al., 1982; Kelemen et al., 2003). constrained because the earthquake occurred to no coupling in the eastern part of the Andre- The subduction of the Amlia fracture zone prior to the deployment of digital broadband anof block. The boundary between the strongly has infl uenced the local seismicity. Most nota- seismometers. Johnson and Satake (1993) coupled and weakly coupled regions coincides bly, since the 1960s, there have been fewer inverted tsunami waveforms to determine that approximately with the Amlia fracture zone. M >5 thrust earthquakes on the plate boundary the largest slip patches occurred west of the GPS strain measurements show that, for exam- east of the Amlia fracture zone, from 173°W Amlia fracture zone. Few aftershocks occurred ple, sites on Atka Island move ~5 mm/yr toward to ~171°W, than have occurred on adjacent and little moment was released east of Amlia the southwest, whereas sites on Adak Island segments of the arc (Fig. 3; Ekstrom and Eng- fracture zone, and thus rupture propagated move 10–15 mm/yr toward the northwest (Frey- dahl, 1989). Both the axis of volcanism and the across the Amlia fracture zone with reduced mueller et al., 2008). The area of the megathrust Wadati-Benioff zone are offset across the Amlia moment release. More recently, the Mw 8.0 in that ruptured in the 1986 Andreanof earthquake fracture zone (House and Jacob, 1983). Based 1986 (Hwang and Kanamori, 1986; Engdahl and the area of highest moment release during on stress inversions of focal mechanisms, Lu et al., 1989) and Mw 7.9 in 1996 (Kisslinger the 1957 Andreanof earthquake both occurred and Wyss (1996) indicated that there are major and Kikuchi, 1997) megathrust earthquakes offshore of Adak Island, which is now nearly changes in stress directions across the fracture occurred beneath the Andreanof Islands, with all 100% locked (Cross and Freymueller, 2007). zone. Although it is apparent that the subduction the moment released west of the Amlia fracture of the Amlia fracture zone affects seismicity, zone during these events. DATA it apparently did not form a barrier to rupture In geodetic studies of the Aleutian Islands, during the 9 March 1957, Mw 8.6 Andreanof Cross and Freymueller (2007) and Freymuel- In this study, we reinterpreted multichannel earthquake, the fourth largest earthquake to ler et al. (2008) used global positioning system seismic-refl ection (MCS) data collected in 1980

180

140 220 200 160 120

100 80 SeguamSeguam 60 AmuktaAmukta AtkaAtka

AmliaAmlia 40 TTanagaanagaanaga 52°N AdakAdak 19961996 19861986

20 19571957

Aleutian Trench

50 KM

178°W 176°W 174°W 172°W 50°N

Figure 3. Earthquakes are for Global Centroid Moment Tensor (CMT) thrust solutions of M 5–6 (yellow), M 6–7 (orange), M 7–8 (green), and M >8 (red) dating from 1976 to 2010 (http://www.globalcmt.org/). The locations of signiﬁ cant earthquake epicenters (1957, 1986, and 1996) are labeled. The depth to the top of the subducting Paciﬁ c plate (in km) is shown by red lines (Hayes et al., 2012). Note the shallow depth of the plate beneath Adak Island.

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and 1981 by the U.S. Geological Survey off- INTERPRETATIONS as evident in bathymetry off of Seguam Island shore of the Andreanof Islands (Fig. 1). These (Fig. 1; Geist et al., 1988). profi les were collected using a fi ve air-gun, The focus of this paper is on the most recent Between Adak and Atka Islands, the Aleutian 2000 psi tuned source array with a total vol- phase of tectonism as expressed in the Pliocene Terrace forearc basin shows a typical forearc ume of 1315 in3 (0.0215 m3) producing shots and Quaternary deposits of the forearc basin. An basin confi guration with evidence for progres- with 50 m spacing. Data were recorded using a outer-arc high formed as the result of accretion sive uplift of an outer arc forming the seaward 2.4-km-long, 24 channel hydrophone streamer of sediment beneath the arc massif along a pri- side of the forearc basin (Figs. 4–6). North of and a GUS 4200 digital recorder. We also uti- mary splay fault, which created accommodation Hawley Ridge, the geometry of the Pliocene– lized a seismic-refl ection profi le collected space for the deposition of forearc basin sedi- Quaternary horizons indicates that the outer-arc by a joint effort of the Woods Hole Oceano- ment (Harbert et al., 1986). Additional accom- high has been uplifting actively throughout this graphic Institution, U.S. Geological Survey, and modation space was created by the progressive time period; in particular, recent uplift is evident Lamont-Doherty Earth Observatory on the R/V uplift of the outer-arc high in response to move- just north (arcward) of DSDP site 186 (Fig. 4). Maurice Ewing in 1994 (1232E994 on Fig. ment on the splay fault. Although the forearc The arcward edge of the forearc basin generally 1). This profi le was collected in 1994 using an basin initially formed in response to accretion shows onlap of basin sediment onto the older 8400 in3 (0.138 m3) air-gun array source and a of sediment at the trench and formation of an middle series and arc framework rock, with 160 channel, 4-km-long multichannel hydro- outer-arc high, it is evident that the present little deformation. However, the western lines phone streamer. All MCS data were migrated structure and sedimentary architecture of the shows that strata along the northern edge of using a poststack fi nite-element migration algo- forearc basin vary substantially along strike. In the forearc basin dip toward the trench, which, rithm with the migration velocities chosen from particular, the basin shape, depositional style, combined with the arcward dip of outer forearc constant-velocity migration panels. A band-pass and syn- and postdepositional deformation basin strata, show evidence of additional short- fi lter (5–40 Hz) and automatic gain control were are notably different on either side of 173°W, ening across the margin (Figs. 5 and 6). The applied to the data. where the Amlia fracture zone is subducting. westernmost profi le shows a relatively narrow Age control for refl ectors within the forearc Stratigraphic horizons that are correlated across basin geometry with ample remaining accom- basin was determined by a direct tie of refl ec- the forearc basin and tied to DSDP site 186 for modation space, and arcward migration of the tors on profi les 7L981 and 10L981 to DSDP age control are used to describe how the forearc depocenter with time (Fig. 6). site 186, drilled into a condensed section of basin has evolved through space and time on Near the intersection of the Amlia fracture the outer forearc basin on the antiformal outer either side of the fracture zone. zone with the Aleutian Terrace, there is a transi- arc (Fig. 1; Scholl and Creager, 1973; Scholl et Forearc basin depocenters lie beneath the tion between the style of forearc basin deforma- al., 1987). DSDP site 186 penetrated to a depth Aleutian Terrace, a prominent geomorphic tion observed east and west of the fracture zone below seafl oor of 926 m, with 143 m of core feature that lies between the summit platform (Fig. 7). Here, the seaward edge of the basin recovery. The core at site 186 had a basal age and trench at water depths of 3–4 km (Fig. 1). has been truncated by extensional and/or strike- of late Miocene to early Pliocene; displaced Although the terrace is a relatively fl at feature, slip deformation along the Hawley Ridge shear blocks of middle Miocene sediment occurred it is punctuated by bathymetric highs and lows, zone. As a result, the forearc basin is perched within the Upper Pliocene section (Scholl including Atka Basin and Hawley Ridge (Fig. with little accommodation space, but with sub- and Creager, 1973). The depths to several key 1). Atka Basin lies immediately west of the stantially less relief at the seaward side of the stratigraphic horizons, including the base of northward projection of the Amlia fracture zone basin than is evident in profi les to the east. East the Lower Pliocene (5.3 Ma), top of the Lower and is the largest subbasin (80 km by ~35 km) of the Amlia fracture zone, normal offset of Pliocene (3.6 Ma), top of the Upper Pliocene within this part of the Aleutian Terrace forearc Pliocene through Holocene refl ectors at the sea- (2.6 Ma), and top of the Lower Pleistocene basin (Fig. 1). West of Atka Basin, the Aleu- fl oor has resulted in the collapse of the seaward (800 ka), were determined by diagnostic fos- tian Terrace is bordered at its southern end by side of the forearc basin (Figs. 8 and 9). This sil assemblages including diatoms, radiolarian, Hawley Ridge, a prominent outer-arc high over extensional collapse of the forearc with as much and foraminifera analyzed by the DSDP ship- 200 km long that extends from western Atka as 1800 m of relief has effectively transformed board party (Scholl and Creager, 1973). Depths Island to west of Adak Island (Fig. 1). Hawley the forearc basin into a perched basin with to these horizons were converted to two-way Ridge stands ~1000 m above the Aleutian Ter- little to no remaining accommodation space. traveltime using P-wave velocities measured race, with the maximum height of the ridge off In the area where the Aleutian Terrace shal- on the core (1.65–2 km/s; Creager and Scholl, Adak Island; there is no prominent outer-arc lows south of Seguam Island, horst and graben 1973). We used stratigraphic horizons extrap- high along the seaward edge of the terrace south structures deform uplifted basin sediment (Fig. olated from DSDP site 186 to generate iso- and east of Atka Basin. North of Hawley Ridge, 9). A subordinate (trench-slope) basin occurs at pach maps defi ning sediment thickness in the the seaward edge of the summit platform has the seaward side of the collapsing forearc that forearc basin for each time interval. Sediment been steepened from near horizontal to dipping is narrower than the main forearc basin (5– thicknesses were converted from two-way south as much as 6°. This deformation of the arc 10 km compared to 40–50 km) and contains traveltime to depth using a velocity of 2 km/s, platform is most evident off Adak Island, where much thinner sedimentary fi ll (<1 km compared consistent with refraction velocities measured the outer edge of the arc platform is now located to 2–4 km). The location of the normal-fault– over the forearc basin and interval velocities at a depth of 1300 m. East of the location where bounded trench-slope basin approximately calculated from MCS stacking velocities; the the Amlia fracture zone is now subducting coincides with the location where the Hawley velocity data available did not justify a more (south of Seguam and eastern Amlia Islands), Ridge shear zone intersects the eastern part of detailed velocity model. The isopach data were the Aleutian Terrace shallows in elevation to a the study area (Ryan and Scholl, 1989). gridded using a radial basis function over a few hundred meters higher than the terrace ele- Although forearc basin deformation is domi- 20 km by 50 km window oriented along the vation west of the Amlia fracture zone (Fig. 1), nated by normal faulting east of the Amlia axis of the forearc basin. and the arc massif has rotated into the forearc, fracture zone, lower Pliocene refl ectors show

1258 Geosphere, December 2012

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/8/6/1254/3344859/1254.pdf by guest on 30 September 2021 Inﬂ uence of the Amlia fracture zone on the evolution of the Aleutian Terrace forearc basin S 1 8 9 L9 L 0 1 10L981 and orange— Dark blue—base of DSDP 186 Z S R 6 HRSZ H TWTT—two-way traveltime; 1. le is shown in Figure 8 1

P D S DSDP 186 D ectors as the result of uplift of the outer-arc high. Transparent yellow unit denotes Transparent high. of uplift the outer-arc ectors as the result V.E. = 6.8 V.E. V.E. = 4.3 V.E. e l p i t l u Multiple M 5 km 10 km le 10L981 crosses forearc at Deep Sea Drilling Project (DSDP) site 186. HRSZ—Hawley Ridge shear zone. (B) Close-up of area zone. (B) Close-up of area (DSDP) site 186. HRSZ—Hawley Ridge shear at Deep Sea Drilling Project forearc le 10L981 crosses 8 9 6 7

B TWTT (s) TWTT N

8 4 2 6

0 14 12 TWTT (s) TWTT 10 ector within Upper Pleistocene (too shallow to be tied directly to DSDP site). Location of profi to DSDP Pleistocene (too shallow to be tied directly within Upper ector A Figure 4. (A) Profi Figure basin refl forearc dip of outer A. Note progressive shown in box Pleistocene; of Lower Pliocene; green—top Pliocene; light blue—top of Upper of Lower basin; red-brown—top forearc upper-series refl exaggeration. V.E.—vertical unusually thick mass transport deposits; the yellow arrow points to the seaward extent of more recent mass transport deposits. recent points to the seaward extent of more unusually thick mass transport deposits; the yellow arrow

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/8/6/1254/3344859/1254.pdf by guest on 30 September 2021 Ryan et al. S 6L981 Trench light blue—top arent yellow unit arent HAWLEY RIDGE HRSZ BACK THRUST ector within Upper Pleistocene (too shallow to be tied directly to Deep Sea Pleistocene (too shallow to be tied directly within Upper ector V.E. = 5.2 V.E. V.E. = 2.9 V.E. Multiple 5 km le is shown in Figure 1. TWTT—two-way traveltime; V.E.—vertical exaggeration. V.E.—vertical TWTT—two-way traveltime; 1. le is shown in Figure 10 km le 6L981 crosses Hawley Ridge in the western sector of the Andreanof block. HRSZ—Hawley Ridge shear zone. (B) Close-up of block. HRSZ—Hawley Ridge shear Andreanof of the Hawley Ridge in the western sector le 6L981 crosses

8 7 9 5 6 B (s) TWTT N

8 4 2 6

0 14 12 TWTT (s) TWTT 10 A Figure 5. (A) Profi Figure Pliocene; of Lower basin; red-brown—top forearc denotes unusually thick mass transport deposits. Dark blue—base of upper-series Pleistocene; and orange—refl of Lower Pliocene; green—top of Upper site). Location of profi Drilling Project area shown in box in A. Note back thrust deforming the entire forearc basin section at the arcward edge of Hawley Ridge. Transp edge of Hawley Ridge. basin section at the arcward forearc A. Note back thrust deforming the entire shown in box area

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S 1 8 98 9 L 5 5L981 h d end of the c n ench e r T Trench d-brown—top of d-brown—top D R A W C R A

S P I DIPS ARCWARD D Pleistocene (too shallow to within Upper ector Z S R HRSZ H le is shown in Figure 1. TWTT—two-way traveltime; V.E.—vertical exaggeration. V.E.—vertical TWTT—two-way traveltime; 1. le is shown in Figure V.E. = 5.7 V.E. e l p i t l D u R V.E. = 3.8 V.E. Multiple M A W A E S

S P I 5 km DIPS SEAWARD DIPS SEAWARD D 10 km

le 5L981 crosses Hawley Ridge offshore of Adak Island. The summit platform is tilted down toward the trench (see also Fig. 1). The summit platform is tilted down toward the trench Adak Island. of Hawley Ridge offshore le 5L981 crosses

5 7 8 6 TWTT (s) TWTT B N

8 4 2 6

0 14 12 TWTT (s) TWTT 10 A HRSZ—Hawley Ridge shear zone. (B) Close-up of area shown in box in A. Note seaward tilt of forearc basin sediment at the arcwar A. Note seaward tilt of forearc shown in box zone. (B) Close-up of area HRSZ—Hawley Ridge shear basin; re forearc basin. Dark blue—base of upper-series imaged within the forearc basin. No mass transport deposits are forearc Pleistocene; and orange—refl of Lower Pliocene; green—top Pliocene; light blue—top of Upper Lower site). Location of profi to Deep Sea Drilling Project be tied directly Figure 6. (A) Profi Figure

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/8/6/1254/3344859/1254.pdf by guest on 30 September 2021 Ryan et al. up S le is 12L981 Trench n—top of Lower n—top of Lower thick mass transport HRSZ TRUNCATED OUTER FOREARC BASIN HRSZ V.E. = 4 V.E. Multiple V.E. = 9.2 V.E. 10 km 5 km ector within Upper Pleistocene (too shallow to be tied directly to Deep Sea Drilling Project site). Location of proﬁ to Deep Sea Drilling Project Pleistocene (too shallow to be tied directly within Upper ector le 12L981 crosses the forearc just west of its intersection with Amlia fracture zone. HRSZ—Hawley Ridge shear zone. (B) Close- zone. HRSZ—Hawley Ridge shear Amlia fracture just west of its intersection with the forearc le 12L981 crosses 8 7 9 6

B TWTT (s) TWTT N

8 4 2 6

0 14 12 TWTT (s) TWTT 10 A shown in Figure 1. TWTT—two-way traveltime; V.E.—vertical exaggeration. V.E.—vertical TWTT—two-way traveltime; 1. shown in Figure of area shown in box in A. Note truncation and removal of outer forearc basin strata. Transparent yellow units denote unusually Transparent basin strata. forearc of outer A. Note truncation and removal shown in box of area deposits. Dark blue—base of upper-series forearc basin; red-brown—top of Lower Pliocene; light blue—top of Upper Pliocene; gree Pliocene; light blue—top of Upper of Lower basin; red-brown—top forearc deposits. Dark blue—base of upper-series Pleistocene; and orange—reﬂ Figure 7. (A) Proﬁ Figure

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/8/6/1254/3344859/1254.pdf by guest on 30 September 2021 Inﬂ uence of the Amlia fracture zone on the evolution of the Aleutian Terrace forearc basin S h c 0 n 8 ench e 5 r L5 L T Trench 3 1 13L580 brown— top of brown— pse of outer forearc forearc pse of outer attened to the orange horizon. Z S R HRSZ H Pleistocene (too shallow to within Upper ector T Z E S S R F F HRSZ H S O T

L L U A A M FAULTS FAULTS F R O NORMAL OFFSET N le is shown in Figure 1. Figure 11 shows area of B ﬂ shows area 11 1. Figure le is shown in Figure e l p i t l u Multiple Multiple M V.E. = 3.8 V.E. V.E. = 12 V.E. 3 km 10 km le 13L580 crosses the forearc east of Amlia fracture zone. HRSZ—Hawley Ridge shear zone. (B) Close-up of area shown in box zone. (B) Close-up of area zone. HRSZ—Hawley Ridge shear Amlia fracture east of the forearc le 13L580 crosses 9 8 7 6 5

B TWTT (s) TWTT N

8 4 2 6

0 14 12 TWTT (s) TWTT 10 Figure 8. (A) Profi Figure basin and lack of accommodation space. Normal offset faults suggest extensional colla tilt of the forearc A. Note trenchward in basin; red- forearc basin. Dark blue—base of upper-series imaged within the forearc basin. No thick mass transport deposits are Pleistocene; and orange—refl of Lower Pliocene; green—top Pliocene; light blue—top of Upper Lower site). Location of profi to Deep Sea Drilling Project be tied directly TWTT—two-way traveltime; V.E.—vertical exaggeration. V.E.—vertical TWTT—two-way traveltime; A

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L L U A A M FAULTS FAULTS F R O NORMAL OFFSET N e l p i t l u Multiple M Multiple V.E. = 2.8 V.E. V.E. = 6.2 V.E. 1 km 10 km le 1232E994 crosses the forearc offshore of Seguam Island. HRSZ—Hawley Ridge shear zone. (B) Close-up of area shown in box zone. (B) Close-up of area of Seguam Island. HRSZ—Hawley Ridge shear offshore the forearc le 1232E994 crosses 7 5 6

B N (s) TWTT 2 6 0 8 4

14 12 10

ector within Upper Pleistocene (too shallow to be tied directly to Deep Sea Drilling Project site). Location of profi to Deep Sea Drilling Project Pleistocene (too shallow to be tied directly within Upper ector TWTT (s) TWTT of upper-series forearc basin; red-brown—top of Lower Pliocene; light blue—top of Upper Pliocene; green—top of Lower Pleistocen of Lower Pliocene; green—top Pliocene; light blue—top of Upper of Lower basin; red-brown—top forearc of upper-series Figure 9. (A) Profi Figure basi imaged within the forearc No mass transport deposits are Terrace. Aleutian of A. Normal offset faults deform elevated area refl exaggeration. V.E.—vertical two-way traveltime; A

1264 Geosphere, December 2012

evidence for an earlier phase of compressional depocenters were generally smaller and more In addition to temporal and spatial changes in deformation. Basin sediments deposited during discontinuous, with little accumulation of sedi- the loci of sediment deposition, there are also the Pliocene show syndepositional uplift of the ment in a forearc basin north of Hawley Ridge. changes in depositional processes as indicated seaward side of the basin with onlap at the north Sedimentation was concentrated beneath the by the character of forearc basin refl ectivity. end of the basin. In addition, Lower Pliocene modern Atka Basin during late Pliocene time, Forearc basin deposits are generally composed strata are higher than coeval strata beneath the with the depocenter having clearly moved of regular, well-defi ned parallel layering that center of the forearc basin, even though this toward the arc. During the early Pleistocene suggests that turbidites constitute most of the pattern was subsequently inverted, with middle (post-Pliocene and pre–ca. 800 ka), overall basin fi ll; turbidites were sampled within the Pleistocene refl ectors now at a lower elevation sediment accumulation was much lower, with outer forearc basin at DSDP site 186 (Scholl and beneath the outer forearc than beneath the cen- little accumulation in localized depocenters Creager, 1973). However, intercalated between ter of the forearc basin (Figs. 8 and 9). We fl at- such as Atka Basin. The depocenter north of the layered refl ectors, there are acoustically tened the refl ection profi le 13L580 on a refl ector Hawley Ridge initially formed during the early chaotic to transparent units that often erode or that has a post–early Pleistocene age (<800 ka; Pleistocene, but only ~400 m of sediment truncate layers below (e.g., Fig. 4). We interpret Fig. 10). Prior to this time, the forearc basin accumulated. A major episode of basin deposi- these units to be mass transport deposits that strata were gently dipping (had not yet been tion began during the late Pleistocene. At this interfi nger with and disrupt turbidite deposition tilted signifi cantly seaward), and the refl ectors time, Hawley Ridge was uplifted, as shown by within the basin. The distribution of mass trans- show the development of an outer-arc high with the elevation of the top of the Lower Pleisto- port deposits beneath the Aleutian Terrace varies arcward-dipping refl ectors to the south, and cene refl ector (Fig. 5). A thickness of more both spatially and temporally along strike. East onlap onto the north. Thus, relatively recently, than 1200 m of sediment was deposited in the of Amlia fracture zone, mass transport deposits the basin was confi gured similar to the forearc depocenter north of the ridge, with an addi- are notably absent from the easternmost profi le basin west of the Amlia fracture zone (e.g., tional 500 m deposited beneath Atka Basin. south of Seguam Island (Fig. 9) and, when pres- compare to Fig. 5). Migration of the depocenter northward toward ent, are thin, areally restricted, and are confi ned Beneath the Aleutian Terrace, multiple dep- the arc, especially in the western part of the to the Pliocene section (Fig. 8). Mass trans- ocenters are evident from isopach maps (Fig study area, is consistent with the active uplift port deposits are much more common west of 11). Depocenters that contain more than 2.4 of Hawley Ridge. Thus, the major uplift of the Amlia fracture zone and are observed on every km of sediment occur beneath Atka Basin and 1-km-high Hawley Ridge combined with the along- and across-strike seismic profi le west of behind (north of) Hawley Ridge (80 km to the deposition of the thickest forearc basin sedi- 173°W, with the exception of the westernmost northwest of Atka Basin). In general, forearc ment all occurred relatively recently within the line (Fig. 6). Commonly, one-third to one-half basin depocenters migrate north and west with last 800 k.y. By middle Pleistocene time, the of the basin-fi ll thickness has stratifi cation dis- time; forearc basin strata east of the Amlia forearc basin east of the Amlia fracture zone rupted by mass transport deposits, which in fracture zone are generally deposited farther had been inverted and had little remaining places are thicker at the arcward (north) side of seaward and are older than the deposits fur- accommodation space, and it has accumulated the basin than at the seaward side (e.g., Figs. 5 ther west (Fig. 11). During the early Pliocene, little sediment since that time. and 7). Three of the profi les that cross the entire

N S 113L5803L5L580

Figure 10. Profi le 13L580 fl attened on refl ector within the Upper Pleistocene (orange horizon in Fig. 8). Once fl attened, the refl ectors at the arcward end of the basin onlap onto older strata, and refl ectors at seaward end are tilted arcward above an outer-arc antiform. TWTT—two-way traveltime.

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400 4 400 0

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N ° 1 5 51°N 51°N 4 40040 4 40040 N ° 2 52°N 5 52°N N ° 2 52°N 5 52°N N ° 1 51°N 5 51°N dashed white line shows the location of the present-day Atka Basin. HRSZ—Hawley Ridge shear zone; AT—Aleutian Trench. AT—Aleutian zone; Atka Basin. HRSZ—Hawley Ridge shear dashed white line shows the location of present-day Figure 11. Isopach maps of forearc basin strata for the Lower Pliocene (5.3–3.6 Ma), Upper Pliocene (3.6–2.6 Ma), Lower Pleisto Pliocene (3.6–2.6 Ma), Lower Pliocene (5.3–3.6 Ma), Upper the Lower basin strata for Isopach maps of forearc 11. Figure based on ty are Thickness is in meters assuming a sediment velocity of 2000 m/s. Interpretations tocene–Holocene (post–0.8 Ma). site 186 to multichannel (as shown in Figs. 4–9) and single-channel seismic-reﬂ Sea Drilling Project

1266 Geosphere, December 2012

width of the forearc show mass transport depos- INFLUENCE OF AMLIA FRACTURE fracture zones, changed abruptly, as indicated its composed of 400–450-m-thick packages of ZONE ON FOREARC BASIN by a remnant of the dead Kula-Pacifi c spread- acoustically chaotic refl ectors (Figs. 4, 5, and EVOLUTION ing center captured south of the Aleutian Trench 7). Although these deposits may represent amal- (Lonsdale, 1988). Between magnetic anomalies gamated events and thus may not have formed We used the patterns of forearc sediment 24 and 24.2, the orientation of the Amlia frac- at one specifi c time, the deposits appear to be deposition and deformation to interpret cen- ture zone changed from approximately N42°W confi ned to specifi c time intervals. Line L91281 tral Aleutian tectonic history in detail. A major (current Pacifi c–North America relative plate (Fig. 7) is the only profi le to show a relatively infl uence in the evolution of the forearc basin is motion) to approximately N-S (current orienta- thick mass transport deposit (~420 m) of Plio- the interaction of the Amlia fracture zone with tion of the Amlia fracture zone south of Aleu- cene age, with the majority of other thick mass the margin. The Pacifi c–Kula–North American tian Trench) (Fig. 12). It is not known how transport deposits deposited during the Quater- plate reconstructions of Lonsdale (1988) were this rotation occurred, but it is likely that the nary. The thickest mass transport deposits off used to determine the location of the Amlia Kula-Pacifi c spreading center was segmented Amlia Island (Figs. 4 and 7) were deposited fracture zone with respect to the Aleutian into shorter axes, as evidenced by irregular, prior to the horizon we correlated within the late Trench through time. Because the Pacifi c–North oblique fracture zones west of the Amlia frac- Pleistocene; on the westernmost profi le where America relative plate motion has been constant ture zone (Lonsdale, 1988). In Lonsdale’s pre- we imaged mass transport deposits (Fig. 5), the since 11 Ma (Atwater and Stock, 1998), we ferred reconstruction, the Amlia fracture zone is thickest deposit is younger than that horizon. used the current relative plate motion near the one of the few fracture zones to persist through Based on available data, the distribution of the Amlia fracture zone (51°N, 173°W) of N42°W this plate reorganization. Kula-Pacifi c spread- thickest mass transport deposits appears to have at 72 mm/yr (DeMets et al., 2010) in our recon- ing ceased at ca. 43 Ma (anomaly 18r), and the migrated to the west with time, but not as far structions. At ca. 56–55 Ma, the orientation of end of the Amlia fracture zone adjacent to the west as Adak Island. Pacifi c spreading fabric, including associated spreading center would have begun to subduct

179°W179°W 177°W177°W 175°W175°W 173°W173°W 171°W171°W 169°W169°W 21 21 10110000 50 0 1001 00 km 18 (FSC) N °

4 23 54°N 5

AFZ 23 21 24.1 21

24.1 24.2 N ° 2 52°N 5 25 23 26 23 24.1 24.1 3.65.3 10.4 Ma 27 24.2 2.1 2.6 0.8 28

N 29 ° 0 50°N 5 25 72 mm/yr 25 30 26 26

Figure 12. Lonsdale’s (1988) preferred reconstruction of the Amlia Fracture Zone (AFZ) and magnetic anomalies 18 (FSC—fossil spreading center) through 31 (see also Atwater, 1989). The locations where the Amlia fracture zone intersected the Aleutian trench back through time are shown by green dots. The change in orientation of the trench axis occurs between the dots labeled 3.6 Ma and 2.6 Ma.

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at ca. 10 Ma at a location 190 km east of its pres- and only migrated along the trench for a dis- faults within the forearc and dip-slip along the ent location (Figs. 12 and 13). tance of ~10 km, owing to the slight obliquity megathrust occurs in the central Aleutian sub- The rate at which the Amlia fracture zone of the trench axis with respect to the plate vec- duction zone. In general, if obliquity along a sweeps along the Aleutian arc has increased tor (Fig. 12). As the trend of the subducting subduction zone is greater than ~30°, slip is through time, not only as a result of changes Amlia fracture zone changed to a more north- generally partitioned between slip along the in the orientation of the fracture zone itself (as erly orientation (Lonsdale, 1988), the rate of megathrust and strike-slip faults in the upper discussed already), but also owing to along-arc along-arc motion increased (Fig. 12). Although plate (McCaffrey, 1992). Complete slip parti- changes in the orientation of the trench axis the change in orientation of the Amlia fracture tioning would result in subduction occurring that affect the obliquity of plate motion (Fig. zone is shown as a gentle bend in Figure 12, it perpendicular to the trench axis, with faults 12). Although the Aleutian Trench is generally is not known exactly how the change in orienta- within the upper plate accommodating all of considered to be arcuate, along the central Aleu- tion from N44°W (anomaly 23 and younger) to the along-arc motion. Ekstrom and Engdahl tians, the trench is composed of sections that N2°W (anomaly 25 and older) occurred. Anom- (1989) estimated that for the central Aleutian are quite linear. East of ~171°W, the Aleutian aly 24.1 along the Amlia fracture zone would subduction zone, ~60% of the along-arc slip is trench trends N69°W over a distance of 750 km have arrived at the Aleutian Trench at about partitioned to strike-slip faults in the upper plate to near Unimak Pass, whereas west of ~171°W, the same time as, or slightly after the arrival of based on the average discrepancy between slip the trench is oriented at N87°W. This results in the Amlia fracture zone at the change in trench vector and plate motion between 172°W and an increase of obliquity of 18° over a relatively orientation. The intersection of the Amlia frac- 179°W. The strike-slip faults in the upper plate short distance. East of the change in trench ori- ture zone at anomaly 25 would have arrived at would then take up the 60% of the along-arc entation, the relative plate motion resolves into the trench at 2.1 Ma near a location along the right-lateral motion or ~27 km/m.y. In the upper 26 mm/yr along strike and 67 mm/yr perpen- trench south of Seguam Island, 90 km east of the plate, strike-slip faults are located both along the dicular to strike. West of the change, the plate fracture zone’s present position (Fig. 12). It has line of volcanism on the Aleutian Ridge and in motion resolves into 48 mm/yr along strike and only been in about the last 2 m.y. that the Amlia the forearc along the Hawley Ridge shear zone 53 mm/yr perpendicular to strike. The Amlia fracture zone has propagated to the west in its (Fig. 1). How much of the 27 km/m.y. slip is fracture zone would have arrived at this change present conﬁ guration and rate. partitioned to the Hawley Ridge shear zone will in trench orientation at ca. 3.6 Ma (Fig. 12). While the Amlia fracture zone propagates to determine the rate at which the Amlia fracture Prior to the Pliocene (5.3 Ma), the position the west, its location with respect to the Aleu- zone propagates along the arc with respect to the of the Amlia fracture zone with respect to the tian Terrace forearc basin is also a function of forearc basin. Since we do not know how this trench would have been relatively stationary whether slip partitioning between strike-slip slip is partitioned, we cannot accurately restore

171°E 175°E 179°E 177°W 173°W 169°W 165°W

54°N 200100 0 200 km 52°N

18r 50°N

AFZ 21 18r 18r 21

SFZ 48°N 18r (FSC) 23 23 21

Figure 13. Reconstruction of the Paciﬁ c plate back to 10 Ma when the Kula Ridge (18r [FSC—fossil spreading center]) ﬁ rst arrived at the Aleutian Trench based on Lonsdale (1988). It is assumed that over this time period, the plate motion has been constant (Atwater and Stock, 1998). The orientation of relative plate motion (shown by gray arrow) is close to parallel to the orientation of the Amlia fracture zone between anomalies 21 and 18r.

1268 Geosphere, December 2012

the location of the forearc basin depocenters fl exing within the block (Geist et al., 1988). The We suggest that a slab tear at the Amlia frac- back through time. However, prior to 2 Ma, internal deformation of the Andreanof block is ture zone could be facilitated by variations in the when arc-parallel strike-slip motion accelerated, manifested by west-directed shortening beneath along-arc component of motion across the frac- the locations of the depocenters would have the Aleutian Terrace (Fig. 14) and perhaps also ture zone, which could impart additional stresses been relatively static. by the presence of a large tholeiitic volcano at on the subducting plate resulting, in a tear along Slip partitioning within the upper plate is Atka Island, possibly formed by faster magma a slab weakness (the fracture zone). The plate accommodated by both strike-slip faulting and ascent facilitated by fractured rock within the west of the change in trench orientation would block rotation (Fig. 1; Geist et al., 1988). The Andreanof block (Kay et al., 1982). A 40 km experience a larger component of along-arc eastern edge of the rotating Andreanof block right-lateral offset perpendicular to the arc of stress than that to the east. Once the slab is off- and the eastern extent of the Hawley Ridge shear the downgoing plate near 173°W (location of set along the fracture zone, the older plate to the zone both occur near the change in orientation Amlia fracture zone) was resolved by House east would be free to roll back, allowing space of the Aleutian Trench, which consequently and Jacob (1983); they suggested that this off- for the rotation of the Aleutian Ridge into the also coincides with the resultant increase in set may be accommodated gradually by fl exure forearc. As House and Jacob (1983) suggested, along-arc motion of the Amlia fracture zone– or by a single tear in the plate. The orientation the plate offset could result in upwelling of trench intersection. Although the Hawley Ridge of a focal mechanism resolved by Ekstrom and asthenospheric material beneath the forearc. shear zone extends along the entire southern Engdahl (1989) is consistent with a tear in the This hypothesis is supported by (1) warm length of the Andreanof block, only the eastern subducting plate. As the offset of the plate along thermal anomalies based on two heat-fl ow Andreanof block has rotated (compare summit the Amlia fracture zone propagates to the west, sites that show values near 100 mW/m2 (Fig. platform east and west of Amlia fracture zone a mass defi ciency between the upper and lower 1; GeoPRISMS Data Portal, 2011) and perhaps in Fig. 1). The difference in the amount of rota- surface of the arc could result in the upwelling associated higher elevation of the Aleutian Ter- tion between the eastern and western portions of asthenosphere along a slab tear (House and race south of Seguam Island (Fig. 1), (2) a zone of the Andreanof is inferred to cause signifi cant Jacob, 1983). of low seismicity for M >5 thrust earthquakes

10 km W V.E. = 19 E 5

TWTT (s) 7

Figure 14. E-W proﬁ le collected parallel to arc that shows shortening perpendicular to the margin in the vicinity of the 1957 and 1986 earthquake epicenters. This forearc high separates the depocenters shown on Figure 10 Upper Pleistocene isopach map. Transparent yellow unit denotes unusually thick mass transport deposits east of the high. TWTT—two-way traveltime; V.E.—vertical exaggeration.

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between Amila and Amukta Islands (Fig. 2), and ridge may be uplifting as the result of a combi- to be suffi cient to overtop the Amlia fracture (3) the altered chemistry of volcanics enriched nation of duplexing and along-arc translation of zone and be accreted to the margin to form a in serpentinite on Seguam Island (Singer et al., outer forearc and arc framework rock (Ryan and small frontal prism (McCarthy and Scholl, 2007), and refraction data showing the mantle Scholl, 1989). 1985). The timing of the initial basin formation wedge located less than 40 km from the trench corresponds to the initiation of Alpine glacia- at the trenchward edge of the elevated terrace SUMMARY OF FOREARC tion in the mountains surrounding the Gulf of (Holbrook et al., 1999). BASIN EVOLUTION Alaska at 5.5–6 Ma (Lagoe et al., 1993; Rea Conversely, the partially detached slab west and Snoeckx, 1995) and subsequent arrival of of the Amlia fracture zone is younger and there- Before the beginning of the Pliocene, the ori- signifi cant amounts of sediment at the central fore somewhat more buoyant, resulting in a entation of the Amlia fracture zone was similar Aleutian Trench. Elevated middle-series and shallowing of the subducting slab (Hayes et al., to the relative plate motions between the Pacifi c Lower Pliocene strata at the seaward side of the 2012), indicating the slab is as shallow as 40 km and North American plate. Thus, the fracture forearc basin indicate uplift of the outer forearc beneath Adak Island (Fig. 2). As shown by Cross zone migrated westward along the trench at a possibly as early as late Miocene through early and Freymueller (2007), the western part of the very low rate and was essentially in place for to middle Pliocene time (Figs. 8 and 9). The old- Andreanof block is an area of strong interplate ~5 m.y. During early Pliocene time, the Amlia est forearc basin sediment was deposited closer coupling. The enhanced coupling west of the fracture zone migrated 33 km to the west along to the trench within several small depocenters, Amlia fracture zone is manifest geomorphically the Aleutian trench to arrive ~150 km east of with no evidence for a major basin depocen- by (1) the uplift of the outer-arc high, Hawley its present location by the end of the early Plio- ter west of Atka Island (Fig. 11). During the Ridge, and (2) the down-tilted summit platform, cene (Fig. 12). Prior to the early Pliocene, no late Pliocene, deposition occurred within one which is now located at a depth of ~1800 m prominent forearc basin was present along the prominent depocenter located beneath what is (Figs. 1 and 6). The uplift of Hawley Ridge rela- central Aleutian arc. The development of an now Atka Basin (the forearc basin depocenter tive to other segments of the trench is not likely outer-arc high that would create accommoda- has migrated an unknown distance to the west owing to locally greater amounts of sediment tion space for accumulation of basin sediment since the late Pliocene). Late Pliocene forearc accretion because trench sediment thickness is did not begin until the early Pliocene. In order basin deposition was located further to the north modest. Trench sediment is actually thicker east to begin the formation of the Aleutian Terrace, in response to a growing outer-arc high; how- of Amlia fracture zone, where the oldest Pacifi c the amount of sediment transported down the ever, little sediment had yet accumulated in the plate is being subducted (Fig. 15). Instead, the Aleutian Trench from the Gulf of Alaska needed forearc basin west of Atka Island (Fig.11).

175°W175°W 173°W173°W 171°W171°W

AATKATKA SEGUAMSEGSEGUAMAM N ° 2 52°N 5 N ° 1 51°N 5 0 1500150 0 2000200 10001000

500500

5025 0 50k m AFZ

Figure 15. Isopach map of sediment deposited on subducting Pacifi c plate. The thickest sediment in the trench is located immediately east of Amlia fracture zone (AFZ), where over 2 km of sediment have accumulated. Thickness is in meters assuming a sediment velocity of 2000 m/s. Interpretations are based on multichannel and single-channel seismic-refl ection profi les available at http://walrus .wr.usgs.gov/NAMSS/.

1270 Geosphere, December 2012

Changes in forearc basin evolution occurred zone had entirely subducted by 1 Ma, the topog- further decreasing arc coupling. Many stud- during the Quaternary. Relatively little sedi- raphy associated with the subducting Amlia ies have related the location of forearc basins ment accumulated in the forearc basin during fracture zone is relatively simple and would to areas of high slip on the megathrust. How- the early Pleistocene (Fig. 10). This could be the not perhaps have formed a signifi cant barrier ever, features on the downgoing plate, which result of the lack of formation of accommoda- to along-trench transport. Suffi cient sediment are translating parallel to the arc as the result of tion space for forearc basin deposition. One pos- was transported from the Gulf of Alaska down oblique subduction, can temporarily interrupt sibility is that sediment from the Gulf of Alaska the Aleutian Trench axis to overtop the Amlia plate coupling. Thus, for example, the mega- did not reach the central Aleutian Trench at this fracture zone barrier and smooth the rougher thrust beneath the largest forearc basin in the time, resulting in little accretion and uplift of topography west of the Amlia fracture zone; central Aleutians, Atka Basin, is now accumu- the outer-arc high. Beginning in the early Pleis- over 1 km of trench sediment accumulated as lating little strain where the Amlia fracture zone tocene, the rate of the along-arc motion of the far west as Adak Island (Fig. 15). Interplate cou- is subducting (Freymueller et al., 2008). Amlia fracture zone increased, and the majority pling beneath the western Andreanof block may One of the consequences of strong coupling of the “bend” in the fracture zone was subducted be enhanced by the smoothing of the subduct- and the rupture of great earthquakes is the trig- (Fig. 12). The “bend” in the Amlia fracture ing plate topography by sediment in the trench gering of very large massive mass transport zone entered the trench during the late Plio- (e.g., Ruff, 1989). This enhanced coupling has deposits. Not only do mass transport deposits cene, with anomaly 24.1 reaching the trench by resulted in shortening across the forearc basin potentially provide a record of past great earth- 3.6 Ma (Fig. 12). We do not know the confi gu- and an increase in accommodation space. Since quakes, but also the thickness and scale of these ration of the bend of the Amlia fracture zone, coupling is higher west of the Amlia fracture deposits suggest that their failure could have but it is likely that the bathymetric expression of zone, whereas sediment in the trench is thicker been tsunamigenic (e.g., Locat et al., 2004). the fracture zone was complex and could have east of Amlia fracture zone, the absolute amount Although we only have a few profi les that image inhibited along-trench sediment transport. A of sediment deposited in the trench may not be the mass transport deposits, and they can be gen- paucity of trench sediment could have precluded as important to coupling as suffi cient amounts erated by a variety of mechanisms (e.g., Hamp- continued development of accommodation of sediment (e.g., Heuret et al., 2012). In the ton et al., 1996), we do note that the majority space by uplift of the outer-arc high, resulting area where the Amlia fracture zone is currently of mass transport deposits occur within the in no major accumulation of forearc basin sedi- subducting, coupling is low despite the deposi- forearc basin opposite where the summit plat- ment in the early Pleistocene (Fig. 11). tion of thick (>2 km) trench sediment. We sug- form has been cut back and eroded, suggesting Major changes in the patterns of forearc gest that a tear within the subducting Pacifi c removal of signifi cant amounts of material (Fig. basin deposition and deformation occurred in plate along the Amlia fracture zone, allowing for 1). It is therefore possible that a combination of the past 1 m.y. or less. Hawley Ridge, a promi- trench rollback and the rotation of the eastern dynamic loading during a great earthquake com- nent outer-arc high, was uplifted with a forearc Andreanof block into the forearc, reduced the bined with a ready sediment source from the basin depocenter containing >1.4 km of sedi- coupling beneath the eastern Andreanof block. oversteepened and down-tilted summit platform ment forming behind the ridge. East of Haw- As a result, the Aleutian Terrace in the eastern created the conditions to generate these unusu- ley Ridge, additional sediment accumulated Andreanof is locally elevated and no longer ally thick mass transport deposits. Hypothetical beneath Atka Basin. The two depocenters are accumulating a signifi cant amount of forearc submarine mass-fl ow tsunamis were modeled separated by a structural high, which formed as basin sediment. for the upper Aleutian trench slope and indicate a result of shortening oriented perpendicular to that large transoceanic tsunamis can potentially the arc (Fig. 14). East of Atka Basin, the forearc IMPLICATIONS FOR be generated by failures such as these (Waytho- basin has been inverted, with little remaining GREAT EARTHQUAKES mas et al., 2009). accommodation space. The outer forearc basin has been offset by strike-slip faulting along the The area of thickest active forearc basin depo- CONCLUSIONS Hawley Ridge shear zone, with the distal end of sition and most accommodation space is near the forearc basin removed near the intersection Hawley Ridge (Fig. 11). This area is also where Forearc basin sediment of the Andreanof of the forearc with the Amlia fracture zone (Fig. the greatest amount of slip occurred during great block records signifi cant tectonic processes that 7). These changes to the central Aleutian forearc megathrust earthquakes in 1957 and 1986 (Ryan affect the central Aleutian arc. Chief among are ascribed, in part, to a combination of (1) a and Scholl, 1993). The ability of this part of the these is subduction of the Amlia fracture zone. decrease in the periodicity of glaciation known forearc to store elastic strain implies that the Although the fracture zone is too small to cause as the mid-Pleistocene transition (e.g., Clark et forearc is stronger, perhaps owing to the over- substantial uplift and collapse of the forearc al., 2006), and (2) the subduction of the Amlia riding plate being colder (away from slab tear) by its bathymetric relief alone, the contrasting fracture zone, both of which affected sediment and composed of thickened arc framework rock. interplate coupling controls the morphology of supply to the central Aleutian Trench. Plate coupling west of the Amlia fracture zone the forearc and outer-arc high, as well as sedi- Although an increase in ice-rafted debris at is enhanced by the shallower plate dip, result- ment deposition patterns in the basin. As the the beginning of the Quaternary signifi ed an ing from at least partial decoupling of the slab fracture zone migrates along the oblique con- increase in glaciation in the Gulf of Alaska, the across the slab tear, allowing greater buoyancy vergence margin, the outer forearc collapses most signifi cant intensifi cation of glaciation did of the younger slab to the west. Conversely, the extensionally in its wake, leaving a perched not begin until just ca. 1 Ma (Prueher and Rea, forearc east of Amlia fracture zone, which is not forearc basin with no remaining accommoda- 2001). Millennial climatic oscillations resulted presently accumulating strain, may be weak- tion space. The strong coupling west of the in multiple pulses of terrigenous sediment to the ened thermally near the slab tear from upwell- fracture zone results in shortening of forearc Gulf of Alaska as the result of more sustained ing of asthenospheric material. Trench rollback, strata, with ongoing deepening of the basin and sea-level lows (Rea and Snoeckx, 1995; Berger with concomitant upper-plate normal faulting, increasing accommodation space. Climate also et al., 2008). Since the bend in the Amlia fracture causes the terrace to collapse toward the trench, exerts an infl uence on strong coupling along the

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Journal of Geophysical Research, v. 94, p. 15,481– local seismotectonics: Geophysical Research Letters, megathrust in that suffi cient sediment derived 15,498, doi:10.1029/JB094iB11p15481. v. 24, p. 1883–1886, doi:10.1029/97GL01848. from continental glaciation must be transported Freymueller, J.T., Woodard, H., Cohen, S.C., Cross, R., LaForge, R., and Engdahl, E.R., 1979, Tectonic implica- down the Aleutian Trench to overtop the Amlia Elliott, J., Larsen, C.F., Hreinsdottir, S., and Zweck, C., tions of seismicity in the Adak Canyon region, central 2008, Active deformation processes in Alaska, based Aleutians: Bulletin of the Seismological Society of fracture zone and smooth the rough topogra- on 15 years of GPS measurements, in Freymuller, J.T., America, v. 69, p. 1515–1532. phy west of the fracture zone. As a result of the Haeussler, P.J., Wesson, R., and Ekstrom, G., eds., Lagoe, M.B., Eyles, C.H., Eyles, N., and Hale, C., 1993, Active Tectonics and Seismic Potential of Alaska: Timing of late Cenozoic tidewater glaciation in the strong coupling west of the fracture zone, great American Geophysical Union Geophysical Mono- far North Pacifi c: Geological Society of America tsunamigenic earthquakes are generated, which graph 179, p. 1–42. Bulletin, v. 105, p. 1542–1560, doi:10.1130/0016 can trigger the deposition of thick mass trans- Fuller, C.W., Willett, S.D., and Brandon, M.T., 2006, For- -7606(1993)105<1542:TOLCTG>2.3.CO;2. mation of forearc basins and their infl uence on sub- Lallemant, H.G.A., and Oldow, J.S., 2000, Active dis- port deposits in the forearc basin. duction zone earthquakes: Geology, v. 34, p. 65–68, placement partitioning and arc-parallel exten- doi:10.1130/G21828.1. sion of the Aleutian volcanic arc based on Global ACKNOWLEDGMENTS Geist, E.L., Childs, J., and Scholl, D., 1988, The origin of Positioning System geodesy and kinematic analy- summit basins of the Aleutian ridge: Implications for sis: Geology, v. 28, p. 739–742, doi:10.1130/0091 block rotation of an arc massif: Tectonics, v. 7, p. 327– -7613(2000)28<739:ADPAAE>2.0.CO;2. We thank Lamont-Doherty Earth Observatory for 341, doi:10.1029/TC007i002p00327. Lander, J.F., and Lockridge, P.A., 1989, United States Tsu- use of the Ewing profi le E994-1232 collected by chief GeoPRISMS Data Portal, 2011, GeoPRISMS Data Portal: namis (Including United States Possessions) 1690– scientists John Diebold and Sue McGeary in 1992; http://www.marine-geo.org/portals/geoprisms/. 1988: Boulder, Colorado, National Geophysical Data these data are available through the academic seismic Gulick, S.P.S., Lowe, L.A., Pavlis, T.L., Gardner, J.V., and Center, National Oceanic and Atmospheric Adminis- portal at the University of Texas Institute for Geo- Mayer, L.A., 2007, Geophysical insights into the Tran- tration, 243 p. physics, http://www.ig.utexas.edu/sdc/. We thank Rob sition fault debate: Propagating strike slip in response Lim, E., Eakins, B.W., and Wigley, R., 2009, Coastal Relief Harris at Oregon State University for directing us to to stalling Yakutat block subduction in the Gulf of Model of Southern Alaska: National Geophysical Data the heat-fl ow data available for the central Aleutians. Alaska: Geology, v. 35, p. 763–766, doi:10.1130/ Center, National Environmental Satellite, Data, and G23585A.1. Information Service (NESDIS), National Oceanic and Discussions with Chris Nye, Steve Kirby, Nathan Hampton, M.A., Lee, H.J., and Locat, J., 1996, Submarine Atmospheric Administration, http://www.ngdc.noaa Bangs, and the U.S. Geological Survey Tsunami landslides: Reviews of Geophysics, v. 34, p. 33–59, .gov/mgg/coastal/s_alaska.html. Source Working Group contributed to this study. We doi:10.1029/95RG03287. 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