Journal of Geophysical Research: Solid Earth

The role of farfield tectonic stress in oceanic intraplate deformation, Gulf of Alaska

Authors


Corresponding author: R. S. Reece, Jackson School of Geosciences, The University of Texas at Austin, J.J. Pickle Research Campus, Building 196, 10100 Burnet Road, Austin, TX 78758-4445, USA. (rsreece@utexas.edu)

Abstract

[1] An integration of geophysical data from the Pacific Plate reveals plate bending anomalies, massive intraplate shearing and deformation, and a lack of oceanic crust magnetic lineaments in different regions across the Gulf of Alaska. We argue that farfield stress from the Yakutat Terrane collision with North America is the major driver for these unusual features. Similar plate motion vectors indicate that the Pacific plate and Yakutat Terrane are largely coupled along their boundary, the Transition Fault, with minimal translation. Our study shows that the Pacific Plate subduction angle shallows toward the Yakutat Terrane and supports the theory that the Pacific Plate and Yakutat Terrane maintain coupling along the subducted region of the Transition Fault. We argue that the outboard transfer of collisional stress to the Pacific Plate could have resulted in significant strain in the NE corner of the Pacific Plate, which created pathways for igneous sill formation just above the Pacific Plate crust in the Surveyor Fan. A shift in Pacific Plate motion during the late Miocene altered the Yakutat collision with North America, changing the stress transfer regime and potentially terminating associated strain in the NE corner of the Pacific Plate. The collision further intensified as the thickest portion of the Yakutat Terrane began to subduct during the Pleistocene, possibly providing the impetus for the creation of the Gulf of Alaska Shear Zone, a > 200 km zone of intraplate strike-slip faults that extend from the Transition Fault out into the Pacific Plate. This study highlights the importance of farfield stress from complex tectonic regimes in consideration of large-scale oceanic intraplate deformation.

1 Introduction

[2] Farfield tectonic stress has been shown to cause significant strain hundreds of kilometers from the source in locations such as the Indo-Australian Plate [Bull et al., 2010; Deplus et al., 1998], and in the North American and Pacific Plates at the Gulf of Alaska [Choy and McGarr, 2002; Mackey et al., 1997; Mazzotti and Hyndman, 2002]. Stress in the Indo-Australian Plate is believed to originate at the nearby Indian Plate collision with the Eurasian Plate. In the Gulf of Alaska, stress transferred to neighboring plates is likely a result of the Yakutat Terrane collision with North America. The Yakutat Terrane is a 15–30 km thick igneous plateau in the northeastern Pacific Ocean [Christeson et al., 2010; Worthington et al., 2012] that began subducting at a low angle beneath North America during the late Eocene to early Oligocene [Finzel et al., 2011]. The thickened Yakutat crust is resistant to subduction compared to the normal oceanic crust of the Pacific Plate. The Pacific-Yakutat plate boundary is the west-northwest striking Transition Fault, a dextral transform boundary at the base of the Yakutat continental slope (Figure 1) [Gulick et al., 2007]. While the Pacific Plate subducts beneath North America at the Aleutian Trench along a northwest vector (~345°) of about 50 mm/yr [Doubrovine and Tarduno, 2008], the Yakutat Terrane subducts beneath North America at a comparable velocity but slightly more westerly vector (~335°) (Figure 1, white arrows) [Elliott et al., 2010]. This configuration results in increasing compressional deformation along the Transition Fault toward the northwest, highlighting the spatial complexity of active deformation along the plate boundary [Gulick et al., 2007; Gulick et al., in press].

Figure 1.

Regional tectonic map of the Gulf of Alaska showing magnetic lineaments (EMAG2) [Maus et al., 2009] and regions constrained by this study. Regional setting is shown in inset (bottom left). Note the lack of coherent magnetic lineaments in the NE Gulf block. Black dashed line is the approximate boundary between Yakutat Terrane collision and subduction [Elliott, 2011]. White arrows are plate vectors for the Pacific Plate [Kreemer et al., 2003] and Yakutat Terrane [Elliott, 2011]; plate rates of motion are with respect to North America. Black one-sided arrows infer apparent fault motion. Orange dashed lines represent the unexposed Yakutat Terrane; pink dashed lines represent fracture zones. Blue-white shapes are seamounts. GASZ, Gulf of Alaska Shear Zone; PZ, Pamplona Zone.

[3] From 1987 to 1992, four large-magnitude (Mw 6.8–7.8) intraplate earthquakes and aftershocks occurred in the Pacific Plate in the northern Gulf of Alaska along what is now referred to as the Gulf of Alaska Shear Zone (GASZ) (Figure 2) [Gulick et al., 2007; Pegler and Das, 1996]. Although seismicity in the area of the 5 year earthquake swarm continues today in this intraplate region, little is known about the potential causes. The goal of this study is to explore possible linkages between the ongoing Yakutat collision with North America and the spatial complexity of deformation in the neighboring Pacific Plate. Intraplate seismicity and shearing in oceanic plates of the magnitude observed at the GASZ is rare, but an important component of understanding the full range of tectonic boundary stress transfer. The nature of stress transfer from the plate boundary to intraplate regions was demonstrated on 11 April 2012, when Mw 8.6 and 8.2 earthquakes occurred in the Indo-Australian Plate offshore Sumatra, >100 km from the plate boundary [Royer, 2012; Yue et al., 2012]. The Pacific Plate in the Gulf of Alaska and the Indo-Australian Plate offshore Sumatra both demonstrate that stresses from complex tectonic regimes can propagate far out into neighboring plates, away from the tectonic boundary itself.

Figure 2.

Topographic and bathymetric map showing Gulf of Alaska seismicity from 1973–May 2012, downloaded from the United States Geological Survey/National Earthquake Information Center database. Regional setting is shown in Figure 1 inset (red box). Focal mechanisms are shown for the four major earthquakes of the Gulf of Alaska Shear Zone (GASZ) swarm [Pegler and Das, 1996]. GASZ earthquakes <35 km depth. Bathymetry and topography from ETOPO2v2.

[4] In this study, we integrate newly reprocessed multi-channel seismic data with bathymetry, recorded seismicity, and geologic and geodetic constraints. We identify and assess the role of Pacific Plate crustal structures in accommodating Cenozoic deformation created by farfield stress from the Yakutat Terrane-North America collision. Our purpose is to place Pacific Plate deformation in a global context and to demonstrate the significance of stress transmitted from tectonic boundaries to the outlying oceanic plate.

2 Data and Methods

[5] This study utilizes multi-channel seismic (MCS) data from multiple surveys in the Gulf of Alaska (Figure 3), with significant differences in acquisition parameters and data quality. The two highest quality surveys were both acquired aboard R/V Marcus Langseth, one in 2008, survey MGL0814, performed as part of the National Science Foundation (NSF) Continental Dynamics St. Elias Erosion/tectonics Project (STEEP) [Christeson et al., 2010; Reece et al., 2011; Worthington et al., 2010; Worthington et al., 2012], and the second in 2011, survey MGL1109, collected under the U.S. Extended Continental Shelf program in cooperation with the United States Geological Survey (USGS). The Langseth surveys employed an 8 km, 640 channel streamer, and a 36 bolt airgun array with a 6600 in3 total volume. The USGS collected the remaining surveys in this study in the 1970s and 1980s. The first survey of those was aboard the R/V Samuel Lee in 1978, survey L378EG [Bruns, 1983; Bruns and Carlson, 1987; Bruns et al., 1987; Reece et al., 2011]. The Lee cruise employed a 1326 in3 airgun array and a 24 channel, 2.4 km streamer. The second USGS survey, F689GA, was collected aboard the M/V Farnella in 1989 as part of an effort to map the U.S. Exclusive Economic Zone [Carlson et al., 1990; Carlson et al., 1996; Reece et al., 2011]. The M/V Farnella deployed a 160 in3 airgun source and a two-channel streamer.

Figure 3.

Topographic and bathymetric map showing locations of seismic lines and observations in this study. Regional setting is shown in inset (top left). The NE Gulf block is shown as a red shaded area. Areas of high-resolution bathymetry detailed in Figures 5 and 8 outlined with green boxes. Orientation of thin-line hash marks represents strike of plate bend faults. Seismic lines shown in figures are represented by white lines. Bathymetry and topography from ETOPO2v2.

[6] Processing of the MCS data included bandpass filtering, muting, velocity analysis, normal move-out correction, and stacking. The 1978 data are shown here for the first time with a frequency-wavenumber migration. The 2008 and 2011 data sets additionally include trace regularization, frequency-wavenumber filtering, water-bottom muting, and finite-difference time migration.

[7] To facilitate discussion on different areas of the Pacific Plate, we have assigned appropriate geographic terms to different sectors of the plate in the Gulf of Alaska (Figures 1 and 3). The NE Gulf block is bounded by the Transition Fault to the north, the GASZ to the west, and the 58° Fracture Zone to the south. Its mirror to the west across the GASZ is referred to as the NW Gulf region and is bounded by the Transition Fault to the north, the Aleutian Trench to the west, the GASZ to the east, and by an indeterminate boundary between the Kodiak-Bowie Seamount Chain and the 58° Fracture Zone to the south. The Pacific Plate adjacent to the Aleutian Trench and south of the NW Gulf region is named the SW Gulf region.

[8] We calculated Pacific Plate subduction angle using Wadati-Benioff Zone seismicity down to 35 km depth. We used 35 km as the limiting depth to get a consistent value over a large portion of the subducting plate without including the deeper “rollover” part of the slab. Results of this method are consistent with other measurements of Pacific Plate subduction angle [Masson, 1991].

3 Observations and Interpretation

[9] The seismic-reflection data displays the Pacific Plate oceanic crust (basement) overlain by Surveyor Fan sediment. Within the crust and sediment, we observe three distinct styles of deformation in three areas of the Pacific Plate in the Gulf of Alaska. The first is the variation in plate flexural bending faults in the Pacific Plate and overlying sediment adjacent to the Aleutian Subduction Zone; second is the strike-slip faulting of the GASZ; and third is the deformation associated with collision and subsequent magmatism in the NE Gulf block. We use the observed deformational styles to define the role of farfield tectonic stress in the Cenozoic structural development of the Pacific Plate in the Gulf of Alaska.

3.1 Plate Bending Faults

[10] We observe widespread faulting on seismic images from the Pacific Plate within ~240 km of the Aleutian Trench (Figure 4a), in the SW Gulf region (Figure 3). Many of the faults are associated with basement ridges that protrude up into the sediment cover, with normal faults and subordinate reverse faults breaking through to the seafloor to be expressed as ridges and gullies. Many of the seismic lines have coincident high-resolution bathymetry coverage with which we can observe fault strike (Figure 3). The faults contain slight variations in strike, but are generally striking north at ≤40° angle to the Trench (Figure 5), and parallel to magnetic lineaments (Figure 1). We consistently observe near-trench faults in the sediment overlying Pacific Plate crust on seismic and bathymetric data in the SW Gulf region. However, seismic lines in the NW Gulf region (Figure 3) reveal no faults seaward of the Trench (Figure 4b).

Figure 4.

Seismic-reflection lines showing the variation in plate bend faulting of the Pacific Plate between southwest and northeast areas at the Aleutian Trench in this study area. (a) Line 15 from a 2011 U.S. Extended Continental Shelf Program survey. Predominantly normal plate bend faults are evident in the sediment overlying Pacific Plate crust. (b) Line 11 from 2008 St. Elias Erosion/tectonics Project (STEEP) survey. See Figure 3 for line location. Vertical exaggeration is ~13:1 (assuming 2000 m/s).

Figure 5.

(a and b) 3-D perspective rendering of high-resolution bathymetry data showing predominantly normal plate bend faults at the seafloor near the Aleutian Trench in the Gulf of Alaska. See Figure 3 for grid locations. High-resolution bathymetry from the 2011 U.S. Extended Continental Shelf Program Survey; remaining bathymetry ETOPO2v2. Note that the scale bar is only accurate in the foreground due to perspective view. Fault strike is shown in plain view in Figure 3. Differences in resolution and edge effects from acquisition result in some displayed inconsistencies in the final mosaic images.

[11] The Pacific Plate subduction angle at the Aleutian Trench ranges from ~7–9° west of the region discussed in this study, corroborated by Masson [1991], to 7° at the Kodiak-Bowie Seamount Chain. As noted in section 2, this dip refers to only the shallow portion of the subducting plate, calculated using Wadati-Benioff seismicity for the upper 35 km. Moving to the northeast, Pacific Plate subduction angle consistently shallows until it reaches 4.5° adjacent to the Yakutat Terrane. The shallowing subduction angle of the Pacific Plate is further evidenced by the northeastward increase in arc to trench distance. Arc to trench distance increases from <150 km in the western Aleutians to >450 km adjacent to the Yakutat Terrane.

[12] We interpret the observed faults as plate bend faults associated with Pacific Plate subduction beneath North America, with a likely reactivation of the Pacific Plate spreading fabric. The reactivation of spreading fabric at this angle to the trench, and location and character of the faults is consistent with observations at other trenches globally [Masson, 1991; Ranero et al., 2005]. Plate bend faults are generally observed in subducting plates globally with plate dip as low as 3° [Masson, 1991]. The northeastward decrease in fault population may stand as additional evidence of the northeastward shallowing of Pacific Plate subduction angle associated with Yakutat Terrane subduction. Ranero et al. [2005] observe a similar phenomenon offshore Central America, where shallow angle subduction of the Cocos Ridge reduces plate bend fault occurrence in the surrounding Cocos Plate.

3.2 Plate Deformation and Magmatic Sills

[13] The oceanic crust defining the Pacific Plate in the northern Gulf of Alaska is Eocene to Miocene in age, containing magnetic anomalies 5–22 that range in age from ~10 to 53 Ma [Atwater and Severinghaus, 1989; Naugler and Wageman, 1973].

[14] Seismic lines located in the NE Gulf block image discontinuous, high amplitude reflections within the sediment just above the basement (Figure 6). The reflections are abrupt, step-like, and differ in size and placement within the section. The sedimentary section immediately above the reflectors exhibits numerous faults among discontinuous reflectors of varying amplitude. The faults rise from the basement and appear to be limited to the NE Gulf block, capped by an ~6 Ma horizon established by Reece et al. [2011] (Figure 6a). Although these step-like reflectors are prevalent mostly in the NE Gulf block, some are present in the immediate vicinity west of the GASZ. This seismic facies is evident on all seismic lines in the NE Gulf block with good imaging of the basement and overlying strata (Figure 3). These reflectors are similar in form to those interpreted as magmatic sills offshore Norway [Cartwright and Møller Hansen, 2006; Møller Hansen and Cartwright, 2006] and others verified as magmatic sills by drilling in offshore Newfoundland [Peron-Pinvidic et al., 2010]. Therefore, we interpret these discontinuous reflections in the NE Gulf block as magmatic sills.

Figure 6.

Seismic-reflection lines exhibiting interpreted magmatic sill intrusions in the NE Gulf block. Sills appear as abrupt, step-like reflections above basement. (a) Zoom of sills from Figure 4b. (b) Line 974 from USGS survey L378EG. (c) Seismic-reflection line 2 from 2008 St. Elias Erosion/tectonics Project (STEEP) survey. Green horizon is ~6 Ma [Reece et al., 2011]. See Figure 3 for line locations. Vertical exaggeration (VE) calculated assuming 2000 m/s.

[15] Global magnetic models and data sets such as EMAG2 show the Gulf of Alaska to contain well-defined magnetic lineaments except in the NE Gulf block where no coherent anomalies are observed (Figure 1). Naugler and Wageman [1973] interpreted no anomalies in the NE Gulf block and referred to this region as the “disturbed zone” for its apparent lack of magnetic anomaly reversals. Since that time, the NE Gulf block has been interpreted to contain magnetic isochrons 9–12 [Atwater and Severinghaus, 1989]. Magnetic data from ship tracks within the NE Gulf block reveal minor deflections from zero, but do not compare in magnitude with magnetic anomalies of similarly aged crust in the rest of the Gulf of Alaska. The Surveyor Fan, the deep-sea sedimentary body covering this portion of the Gulf of Alaska seafloor, is in excess of 3 km thick in areas of the NE Gulf block [Reece et al., 2011] and could be perceived to interfere with Pacific Plate magnetic measurements. However, the same sediment thicknesses are present in the Surveyor Fan west of the GASZ, as well as southeast in the Baranof Fan, and both of these areas exhibit clear magnetic lineations, likely precluding sedimentary cover as a hindrance to magnetic measurements.

3.3 Gulf of Alaska Shear Zone Strike-Slip Faults

[16] The GASZ is a north striking zone of predominantly right-lateral deformation [Pegler and Das, 1996] persisting 260 km from the Transition Fault in the north to the Kodiak-Bowie Seamount Chain in the south (Figure 2). The Transition Fault and seamount chain both trend roughly WNW–ESE. Modern seismicity parallels the plate spreading fabric evidenced by magnetic lineaments (Figures 1 and 2). At its northern extent, the GASZ is clearly visible in four seismic-reflection images from separate surveys in 1973, 1989, and 2008 (Figure 7). Select zooms of interpreted GASZ faults in the seismic data are shown at significantly reduced vertical exaggeration to emphasize the nearly vertical nature of the faulting. All seismic lines clearly image the principal fault of the GASZ from Pacific Plate basement up into Surveyor Fan sediment (Figure 7). The seismic data show that the GASZ consists of multiple fault segments (Figure 3, plan view location of faults); and in all images, the faults penetrate upward from the sediment-oceanic basement contact to varying levels within the overlying sediment. Faults in the south appear as single, vertical to subvertical structures, with minimal offset of strata but visible stratal disruption (Figures 7a, 7b, and 7e). However, the northern two imaged faults (Figures 7c and 7d) branch upward in the area of a broad basement high and seafloor ridge (Figure 8). Seismic line crossings of the GASZ do vary from orthogonal to oblique, but only one line (Figure 7d) is very oblique. Due to minimal stratal offset observed in the seismic data, we interpret the faulting to be primarily strike-slip in origin, which is consistent with GASZ focal mechanisms (Figure 2) [Pegler and Das, 1996]. Bathymetric data show a 45 km long, north-trending ridge coincident with the GASZ that is perpendicular to and abutting the Transition Fault (Figure 8). This bathymetric ridge exhibits high relief in the north, higher than 200 m over surrounding topography, and diminishes southward. Proximal to the shelf, a basement high with ~1 s of relief is evident beneath the ridge (Figure 7d). Based on the upward bifurcation of fault segments to the north, as well as the presence of the ridge and basement high, we interpret GASZ faults as increasingly transpressional to the north and as likely responsible for formation of the basement high. Transpression, however, is not well reflected in the modern seismicity, which exhibit only strike-slip focal mechanisms [Pegler and Das, 1996].

Figure 7.

Seismic-reflection lines showing interpreted Gulf of Alaska Shear Zone (GASZ) faults. (a) Line 973 from USGS survey L378EG intersects the GASZ normally. In addition to the GASZ, magmatic sills just above basement are evident on this line. (b) Zoom of the GASZ from Figure 7a at considerably less vertical exaggeration (VE). (c) Zoom from Figure 4b. (d) Line 30 from USGS survey F689GA is a slightly oblique crossing of the GASZ. At this location, the GASZ is associated with a bathymetric ridge and basement high. (e) Line 972 from USGS survey L378EG is an extremely oblique GASZ crossing. See Figure 3 for line locations. VE calculated assuming 2000 m/s.

Figure 8.

3-D perspective rendering of high-resolution bathymetry data showing the seafloor ridge near the intersection of the Gulf of Alaska Shear Zone and the Transition Fault, the Yakutat Terrane-Pacific Plate boundary in the Gulf of Alaska. Bathymetry overlain with Gulf of Alaska Shear Zone (GASZ) seismicity from 1973–May 2012, downloaded from the United States Geological Survey/National Earthquake Information Center database. Transition Fault interpretation modified from Gulick et al. [in press]. Note that the scale bar is only accurate in the foreground due to perspective view. High-resolution bathymetry from Gardner et al. [2006]; remaining topography ETOPO2v2.

[17] Figure 9 displays a seismic-reflection profile that crosses the GASZ in the south, north of and parallel to the west-northwest trending Kodiak-Bowie Seamount Chain. The Pacific Plate crust is much rougher in this region close to the seamounts, compared to the crust in the remainder of the Gulf, modified by volcanism associated with the formation of the Kodiak-Bowie Seamount Chain. No GASZ faults are observed here and historical seismicity on the southern section of the GASZ ends at the approximate location of the seismic line (Figures 2 and 3). The lack of faulting and seismicity in this region suggests termination of the GASZ at the northern end of the Kodiak-Bowie Seamount Chain.

Figure 9.

Seismic-reflection line 13 from a 2011 U.S. Extended Continental Shelf Program survey that crosses the Gulf of Alaska Shear Zone (GASZ) seismicity at its distal extent. No conspicuous GASZ faulting is evident at this location. Seamounts are members of the Kodiak-Bowie Chain; channel is a massive branch of the Chirikof Channel system. See Figure 3 for line location. Vertical exaggeration is ~13:1 (assuming 2000 m/s).

4 Discussion

4.1 Plate Bending Faults

[18] Increasing arc-trench distance and a significantly widened zone of shallow Wadati-Benioff Zone seismicity [Eberhart-Philips et al., 2006] evidences a northeast shallowing of Pacific Plate subduction along the Aleutian Trench. The spatial decrease in plate bend faulting from southwest to northeast observed in this study could be explained by the northwest Pacific acting as a transitional area accommodating a shift in subduction geometry from the higher-angle subduction dip of the Pacific Plate in the SW Gulf region to the lower-angle subduction of the Yakutat Terrane in the north (Figure 3). For this to be possible, the Pacific Plate must be somewhat coupled to the Yakutat Terrane despite having a slightly different plate motion vector [Elliott et al., 2010]. The subducted boundary between the Yakutat Terrane and Pacific Plate, a westward projection of the Transition fault, must exhibit sufficient cohesion in order to maintain contact through subduction. This concept of coupled plates is bolstered by the fact that the 1964 9.2 Mw Good Friday earthquake occurred on the Yakutat-North America subduction interface, yet propagated southwest across the Yakutat-Pacific boundary and >600 km down the Aleutian megathrust [Shennan et al., 2009]. Coupling between the Yakutat and Pacific has implications for more than just plate bending. The Pacific and Yakutat interact in a way such that stress does not appear to localize in the North American Plate above their subducted boundary. No major faults or evidence of strain are present in this area of the subducted boundary, which means that any stress generated at the boundary must be transmitted elsewhere, such as to the unsubducted Transition Fault, the GASZ, or other areas in the North American Plate.

4.2 NE Gulf Block

[19] Why are the magnetic spreading anomalies muted in the NE Gulf block? Minor magnetic deflections do exist, and Pacific crust in the northern Gulf of Alaska is Eocene to Miocene in age, both ruling out the possibility of the Cretaceous long-normal superchron. The surrounding crust is rife with well-defined magnetic lineaments, leaving us to consider the Naugler and Wageman [1973] hypothesis that magnetic anomalies in the triangular region of the NE Gulf block have been disrupted by Cenozoic crustal deformation. However, understanding of Gulf of Alaska tectonics has advanced significantly since the 1970s, and we now have more data and information from which to draw our conclusions. We suggest that the magmatic sills observed in the reflection seismic data (Figure 6) are prevalent throughout the NE Gulf block and may have severely distorted the magnetic signatures of the underlying Pacific Plate crust in this area. Due to presence of thick sedimentary fan sequences throughout the Gulf of Alaska in locations with well-defined magnetic lineaments, we believe that the thick sediment cover is not a factor in the distortion of the NE Gulf block magnetic anomalies.

[20] Several questions remain. Where did the sills come from, and why do they appear to be mostly confined to the NE Gulf block? We hypothesize that the NE Gulf block deformed significantly as a result of the flat-slab subduction and developing collision of the Yakutat Terrane with North America. Flat-slab subduction is shown to begin as long ago as the late Eocene–Early Oligocene [Finzel et al., 2011]. Stress from the collision could have transferred outboard to the Pacific Plate, where the normal oceanic crust formed by tectonic processes may represent a more easily deformed member in comparison to the crust of the Yakutat Terrane, which was formed by large-scale crystallization of mantle melts. The transfer of stress from the collisional zone outward could have subsequently been altered by a shift in plate motion at ~6 Ma when the Pacific Plate shifted to a more orthogonal convergence with North America in southern Alaska [Doubrovine and Tarduno, 2008]. This theory is supported by the significantly deformed sedimentary section above the sills in the NE Gulf block, capped by a ~6 Ma horizon above which there is little to no deformation (Figure 6a). The 58° Fracture Zone, the southern boundary of the NE Gulf block, may have acted as a structural boundary beyond which stress from the collision was significantly dissipated or unable to propagate across. The GASZ acts as the NE Gulf block western boundary, an important point that will be discussed more in the following section. These tectonic features define the boundaries of collisional deformation within the NE Gulf block triangle. We further hypothesize that the deformation itself potentially created pathways for upwelling magma to penetrate the lower crust. The necessity of a pathway for the magma to escape would explain why the sills are largely limited to the NE Gulf block and the extensively deformed oceanic crust within. McNutt et al. [1997] observed a similar phenomenon in the Austral Islands in the South Pacific and concluded that plate structures controlled the location of magma outflow from the mantle.

[21] Concerning magma origins, several possibilities exist, including off-axis ridge magmatism, the Kodiak-Bowie hotspot, and broad-scale mantle upwelling. Off-axis magmatism may be unlikely due to the widespread nature of the sills and the apparent connection between crustal deformation and magma propagation. The Bowie Plume, believed to have contributed to the formation of the Kodiak-Bowie Seamount Chain over the last 24 myr [Chaytor et al., 2007], may have formed a magma chamber off-axis from the main chain volcanism as it traversed the region adjacent to the NE Gulf block. Major Kodiak-Bowie seamounts (Pratt and Welker) are located 150 km and 250 km south of the NE Gulf block (Figure 3) and formed 18 and 15 Ma, respectively [Chaytor et al., 2007; Gulick et al., in press]. For this explanation to be feasible, magmatism would have had to linger for ≥10 myr or be sourced from younger volcanism farther southeast on the seamount chain. In an area as large as the NE Gulf block, magma could also be sourced from broad mantle upwelling or a large zone of upper mantle partial melt, as suggested by McNutt et al. [1997] in the southern Austral Islands. Drill core recovery of sill material and additional seismic data in the NE Gulf block could illuminate the origins of this extensive magmatic province.

4.3 Gulf of Alaska Shear Zone

[22] The seismic-reflection lines provide clear evidence that the GASZ was active long before the 1987 earthquakes began. The basement high and ridge may be the product of protracted fault activity, like that seen at other strike-slip faults evolved over millions of years globally, and may be considered evidence of the relatively long-lived nature of GASZ faulting [Storti et al., 2003]. Since modern focal mechanisms do not reflect transpression, this style of deformation may be more representative of earlier stresses and deformation at the GASZ, and not the processes of present day.

[23] Positioned along an isochron, GASZ faults appear to exploit the abyssal hill fabric of the Pacific Plate, possibly connecting a multitude of relict normal faults that enabled faulting to spread far to the south. Previous studies show stress from tectonic boundaries to cause or influence intraplate strain; this strain frequently takes advantage of preexisting plate weaknesses such as spreading fabric or fracture zones [Deplus et al., 1998; Storti et al., 2003]. Abrupt termination of GASZ seismicity in the south is likely due to the presence of the Kodiak-Bowie Seamount Chain. We propose that the seamount chain volcanism annealed the inherited plate fabric with an influx of new crustal material, potentially erasing major weaknesses in the Pacific Plate as well as the opportunity for propagation beyond the Kodiak-Bowie Seamount Chain.

[24] What are the possible drivers of this intraplate deformation? A recent GPS study shows that Yakutat Terrane interaction with North America can be divided into two distinct regimes [Elliott, 2011]. To the west, the block above the Yakutat-North America interface consists of relatively uniform GPS velocity with wide locked segments that serve as the beginnings of the Aleutian megathrust. In the east, a system of crustal thrust faults that sole into the Yakutat décollement break up the overriding North American Plate into small blocks and mark the collision of Yakutat with North America. The boundary between the two is abrupt, discrete, and is directly inline with the GASZ (Figure 1, black dashed line) [Elliott, 2011].

[25] Simple block tectonic reconstructions of the Gulf of Alaska at 6 Ma and 1 Ma demonstrate that with little translation along the Transition Fault, it would have been possible for the GASZ to maintain a steady position relative to the Yakutat Terrane through time (Figure 10). Recent work defines the Transition Fault as originally active in the Eocene to Oligocene, followed by a long period of dormancy, essentially allowing for protracted coupling of the two plates, until reactivation in the Pleistocene [Gulick et al., 2007; Gulick et al., in press]. Even considering activity since the Pleistocene, translation along the Transition Fault is minimal, <10 mm/yr, due to the similar nature of Yakutat and Pacific plate motion vectors [Elliott et al., 2010; Gulick et al., 2007]. The Pacific Plate appears to have followed the stress regime boundary that initiated on land, contributing to the separation of blocky, deformed Pacific crust east of the GASZ, and smoother, intact crust west of the GASZ. Since the Yakutat Terrane thickens substantially to the east (Figure 10c) [Worthington et al., 2012], the boundary between collision and subduction possibly became more enhanced as the collision progressed and the thickest part of the Yakutat encountered the North American margin during the Pleistocene. The increased stress from this stage of the collision may have been the impetus required to create the 200 km long GASZ in the Pacific. The Plate may have faulted naturally at this location due to the stress boundary between subduction and collision already in place on land, and because of a boundary in plate strength and character after the large-scale formation of sills in the NE Gulf block. Additional drill core and seismic data may help to further explain the role of post-rift magmatism in conditioning the deformation patterns of an oceanic plate in collisional or tectonic boundary settings that transmit significant stress to the surrounding area. Displacement across the GASZ faults is currently unknown, but we speculate that it could be on the same order as the Transition Fault, <10 mm/yr. If this is the case, each side of the fault would have moved <5 km since ~1 Ma. As this strain would likely be distributed throughout the length of the fault, we are not likely to observe this deformation at the resolution of this study.

Figure 10.

Tectonic block reconstruction of the Gulf of Alaska showing components of this study through time. (a) 6 Ma, (b) 1 Ma, and (c) a zoom of the present day configuration with interpretations from this study labeled. Yakutat Terrane extent modified from Eberhart-Philips et al. [2006]; thickness modified from Worthington et al. [2012] and Ferris et al. [2003]. QC-F, Queen Charlotte-Fairweather.

[26] The earthquakes of the 1987–1992 swarm are an unusual series of large magnitude intraplate events. The four main events were all strike slip, the largest being Mw 7.8, in an area with no previously recorded seismicity. Along with the 2012 Mw 8.6 and 8.2 events offshore Sumatra, the GASZ events demonstrate that stresses from complex tectonic regimes can propagate far out into the involved plates, away from the tectonic boundary itself.

5 Conclusions

[27] The collision between the Yakutat Terrane and the North American Plate in the northern Gulf of Alaska appears to have substantially influenced deformation in the neighboring Pacific Plate. Our conclusions are supported by observations in three areas exhibiting three different styles of deformation in the Gulf of Alaska. We observe a drastic northeastward reduction in observed plate bend faults, which matches the trends of the shallowing Pacific Plate subduction angle and a marked increase in arc to trench distance, both also to the northeast, where the Pacific Plate is in contact with Yakutat Terrane. Coupling of the subducted Yakutat Terrane and Pacific Plate likely enabled strain from the 1964 9.2 Mw great Alaska earthquake to propagate across the Yakutat-Pacific plate boundary.

[28] The NE Gulf block contains crustal and lower sedimentary fan deformation accompanied by magmatic sill emplacement. These features are apparently limited by the tectonic boundaries of the NE Gulf block. Deformation of the Pacific Plate crust and overlying sediments in the NE Gulf block was likely caused by stress from the Yakutat Terrane collision with North America, predating ~6 Ma. The magnetic signature of the Pacific Plate in the NE Gulf block is likely dampened by the presence of magmatic sills emplaced in the Pacific Plate crust and overlying sediment.

[29] Finally, we seismically observe large-scale strike-slip faults out into the Pacific Plate colocated with the extreme seismicity of the GASZ. The faults appear to be transpressional at their northern extent, as evidenced by upward fault bifurcation and the presence of basement and seafloor ridges, and as true strike slip at their southern extent.

[30] We interpret the GASZ as a reactivation of plate spreading fabric and a likely offshore extension of the boundary between Yakutat Terrane subduction and collision. The GASZ is a long-lived feature, possibly initiated after the thickest portions of the Yakutat Terrane began to collide with North America during the Pleistocene and enabled by coupling between the Pacific Plate and Yakutat Terrane.

Acknowledgments

[31] The authors thank the crews and science parties of R/V Marcus Langseth cruises MGL0814 and MGL1109 for acquisition of the 2008 and 2011 data sets, and the USGS for assistance with open-source seismic data. Thanks to Ian Norton for important discussion in the formative stages of this work. Thoughtful reviews by Lisa McNeill and Doug Wilson greatly benefitted this manuscript. This project was funded by NSF grant EAR-0408584 and USGS grant G11AC20096 to the University of Texas at Austin. Reece received partial support from the University of Texas Institute for Geophysics (UTIG) Gail White fellowship. This is UTIG contribution # 2565.

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