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Propagating Strike Slip in Response to Stalling Yakutat Block Subduction in the Gulf of Alaska

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Propagating Strike Slip in Response to Stalling Yakutat Block Subduction in the Gulf of Alaska Geophysical insights into the Transition fault debate: Propagating strike slip in response to stalling Yakutat block subduction in the Gulf of Alaska Sean P.S. Gulick* Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin, Lindsay A. Lowe J.J. Pickle Research Campus, Austin, Texas 78758, USA Terry L. Pavlis Department of Geological Sciences, University of Texas at El Paso, El Paso, Texas 79968-0555, USA James V. Gardner Center for Coastal and Ocean Mapping, University of New Hampshire, 24 Colovos Road, Durham, Larry A. Mayer New Hampshire 03824, USA ABSTRACT POS-MV inertial motion unit interfaced with On the basis of faulting mapped on seismic refl ection and bathymetric data, seismicity, cur- a NovAtel OEM2–3151R global positioning rent plate motions, and evidence that the Yakutat block may be anomalously thick, we propose system (GPS) allowed conversion of traveltime a tectonic model for Yakutat-Pacifi c interactions, including the often-debated Transition fault. to depth, including a water-column refraction To the east, deformation associated with the Queen Charlotte–Fairweather fault system is correction, and compensation for roll, pitch, extending offshore, facilitating westward propagation of strike-slip motion along the eastern and yaw. Spacing of individual soundings is segment of the Transition fault. To the west, the oblique-slip Pamplona zone and Transition ~50 m and vertical accuracy is ~0.3%–0.5% of faults merge at an embayment in the continental margin, where a north-south dextral strike- the water depth. slip fault within the Pacifi c plate, illuminated by the 1987–1992 earthquake swarm, intersects In 2004, 1800 km of high-resolution seismic- the Pacifi c-Yakutat tectonic boundary. These fault patterns are consistent with modern plate refl ection profi les were collected in the Gulf of motions and refl ect a plate boundary reorganization that may be caused by resistance to sub- Alaska aboard the R/V Maurice Ewing as an duction by the Yakutat block, a possible moderate-sized oceanic plateau. Integrated Ocean Drilling Program site survey. The sources were dual 45/45 in3 GI (generator/ Keywords: collision, Alaska, Yakutat, oceanic plateau, Transition fault, subduction. injector) airguns with a better than 5 m vertical resolution. Processing included trace regular- INTRODUCTION fault with only minor Pliocene–Pleistocene ization, normal moveout correction, bandpass The Yakutat block in the Gulf of Alaska has motion (Bruns, 1983), a dextral-oblique fault fi ltering, muting, f-k (frequency-wave number) been colliding with the North American plate (Lahr and Plafker, 1980), and a low-angle fi ltering, stacking, water-bottom muting, and in a 600-km-long orogenic belt over ~10 m.y. thrust (e.g., Perez and Jacob, 1980; Plafker fi nite-difference migration. These profi les add (Plafker et al., 1994; Rea and Snoeckx, 1995). et al., 1994; Fletcher and Freymueller, 2003). to thousands of kilometers of basin-scale seis- This collision has resulted in underthrusting of Conversely, its lack of seismicity (Page et al., mic data collected by private industry and the ~600 km of Yakutat crust and has generated a 1989) and local burial by undeformed or U.S. Geological Survey (USGS) (Bruns, 1983, fl at-slab subduction zone with a subhorizontal weakly deformed sediment (Bruns, 1985) sug- 1985; Bruns and Carlson, 1987). Wadati-Benioff zone (Fig. 1) that occupies a gap gest that the Yakutat block is essentially mov- in the Aleutian magmatic arc (e.g., Eberhart- ing with the Pacifi c plate. Observations Phillips et al., 2006). The broad Chugach– The nature of the Transition fault is critical Bathymetry data show linear ridges in the St. Elias orogeny is formed by this collision, and to understanding the Yakutat collision with its seafl oor sediment along the base of the slope includes the highest coastal relief in the world; far-fi eld tectonic effects (Mackey et al., 1997; that separates the Yakutat block from the Pacifi c it is bound to the north by the Denali fault sys- Mazzotti and Hyndman, 2002). We present plate (Fig. 2B1), where the Transition fault is tem and Wrangell volcanic fi eld. To the south, a revised tectonic model for the Transition expected. A single fault trace is observed in the the Pacifi c plate slides in a right-lateral sense fault that uses evidence for an unusually thick southeast, where it truncates a series of small past the North American plate along the Queen Yakutat block, the presence of the 1987–1992 fans at the base of slope for ~100 km. To the Charlotte–Fairweather fault system to the east Gulf of Alaska earthquake sequence, and cur- northwest, near the Pamplona fold-and-thrust and subducts beneath the North American plate rent plate motions to explain seismic and bathy- belt, there are two linear escarpments (including along the Aleutian trench to the west. In between, metric observations of faulting. Yushin Ridge) with signifi cant seafl oor relief that the Pacifi c lithosphere appears to be subdivid- are interpreted as active faults. The outer strand ing, based on a 1987–1992 earthquake swarm. It SEISMIC AND BATHYMETRIC DATA in the northwest appears in line with the single is doing so along a north-south lineament that is Methods strand in the southeast, whereas the inner strand likely reactivated, spreading ridge-parallel fault- In 2005, more than 162,000 km2 of high- lines up with a smaller section of bathymetric ing (Pegler and Das, 1996), that we refer to as resolution (~100 m) multibeam sonar data relief just southeast of the Yakutat sea valley the Gulf of Alaska shear zone (Fig. 1). were collected along the base of the slope in (Fig. 2B). The transition in steepness between Interpretations of existing data on the the Gulf of Alaska in support of a potential slope (~12°) and Surveyor Fan sediments (~2°) Yakutat-Pacifi c boundary, the Transition fault, U.S. submission for an extended continental implies that the base of slope is structurally con- are controversial, including whether the fault shelf (Gardner et al., 2006). These data were trolled. We suggest that these zones of relief are existed during initial Yakutat–North American collected aboard the R/V Kilo Moana, which all part of the Transition fault system. collision. The Transition fault (Fig. 1) has been is equipped with a hull-mounted Kongsberg These observations are consistent with our variably described as a rejuvenated left-lateral EM120 (12 kHz) multi beam echo sounder that remigration of a USGS profi le (Fig. 3) that generates 191 1° × 2° beams over a 150° swath. *E-mail: [email protected] Frequent sound-speed profi les and an Applanix 1Figure 2 is provided on a separate insert. © 2007 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, August August 2007; 2007 v. 35; no. 8; p. 763–766; doi: 10.1130/G23585A.1; 4 fi gures; 1 insert. 763 CDP 200 300 400 500 600 700 800 900 A 0 VE < 8X WRA KEY 10 km ALASKA NGE Kayak line (1985) LL MOUNT North American 1 CHUGAC Plate Major faults Study H MOU AINS Representative minor Area Inactive back thrust NTAINS thrust faults 2 Glacial systems ~3 km Transition fault PACIFIC OCEAN Yakataga Fm. outcrops 3 SAI Depth in meters NT ELIAS MTNS. Published GPS stations Seismic lines (shown) 4 Bagley Icefield Sew (2004) ard Glacier Seismic lines (not shown) traveltime (s) Two-way Bering (2004) 5 Glacier Earthquakes Malaspina Survey boundary Glacier (1979) Mw6.7 6 Mw5.8 7 Mw4.9 Dan Fairweather fault USGS 78 5. Line 965 Mw na ger lo s MigFX .0 p e 7 n ke a Fig. ous River Zone Pam o u PACIFIC PLATE YAKUTAT BLOCK Z q Mw5 th 70 1B Ear 19 W-12-79-EG Figure 3. U.S. Geological Survey (USGS) Kayak Island Pamplona seismic line 965 acquired by Bruns (1985) Zone Zone Yakutat Mw 7.2 Mw7.8 and remigrated at University of Texas 1987 1987 e Block Khitrov Transition fault? Institute of Geophysics using FX migrated uenc Ridge 2 Yushin ( frequency-depth step) post-stack, time 99 -1 Ridge Mw7.6 migration algorithm; see Figures 1 and 2 87 Fig. 2A Lituya Bay EQ 19 f Alaska Seq Alaska f 1958 for location. Note the clear high-angle fault N o f ul Fig. 3 at base of slope that offsets sediments to G utian trench Mw5.8 Mw6.8 Pacific Plate Mw5.4 M 6.4 near the seafl oor and an inactive thrust fault Ale 1992 Cross Sound Sequence landward. CDP—common depth point; VE— (5.6 cm/yr) 1973 Mw7.7 vertical exaggeration. 1988 W and its near-vertical orientation, it is almost cer- Distance in profile (km) B 0 100 200 300 400 500 600 700 800 900 tainly a strike-slip fault (Fig. 2A). 3 3 On all three profi les, the older sediments Mt. McKinley Chugach-St. Elias Mtns. 2 2 beneath the strike-slip fault show convergent BEARR 1 Unsubducted Yakutat Block 1 folding and faulting; thus, the strike slip is a later 0 0 phase (e.g., Fig. 2A). Only ~200 m of sediment VE=3:1 Elevation (km) Elevation -1 -1 were deposited during the interval cut by the strike-slip fault. Holocene sediments, observed 0 0 in depositional lows such as where these profi les -100 -100 are located, as thick as 300 m (Jaeger et al., 1998) -200 -200 and shelf-wide Holocene sedimentation rates Depth (km) VE=3:1 -300 -300 0 100 200 300 400 500 600 700 800 900 estimated to be 7.9 mm/yr (Sheaf et al., 2003) suggest that the 200 m of sediment were depos- Figure 1.
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