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Fault Trends on the Seaward Slope of the Aleutian Trench: Implications For

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Fault Trends on the Seaward Slope of the Aleutian Trench: Implications For JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. B10, 2477, doi:10.1029/2001JB001433, 2003 Fault trends on the seaward slope of the Aleutian Trench: Implications for a laterally changing stress field tied to a westward increase in oblique convergence Carlos A. Mortera-Gutie´rrez Instituto de Geofı´sica, Universidad Nacional Auto´noma de Me´xico, Coyoaca´n, Me´xico David W. Scholl U.S. Geological Survey, Menlo Park, California, USA Richard L. Carlson Department of Geology and Geophysics, Texas A&M University, College Station, Texas, USA Received 1 October 2001; revised 25 March 2003; accepted 11 June 2003; published 16 October 2003. [1] Normal faults along the seaward trench slope (STS) commonly strike parallel to the trench in response to bending of the oceanic plate into the subduction zone. This is not the circumstance for the Aleutian Trench, where the direction of convergence gradually changes westward, from normal to transform motion. GLORIA side-scan sonar images document that the Aleutian STS is dominated by faults striking oblique to the trench, west of 179°E and east of 172°W. These images also show a pattern of east-west trending seafloor faults that are aligned parallel to the spreading fabric defined by magnetic anomalies. The stress-strain field along the STS is divided into two domains west and east, respectively, of 179°E. Over the western domain, STS faults and nodal planes of earthquakes are oriented oblique (9°–46°) to the trench axis and (69°–90°)tothe magnetic fabric. West of 179°E, STS fault strikes change by 36° from the E-W trend of STS where the trench-parallel slip gets larger than its orthogonal component of convergence. This rotation indicates that horizontal stresses along the western domain of the STS are deflected by the increasing obliquity in convergence. An analytical model supports the idea that strikes of STS faults result from a superposition of stresses associated with the dextral shear couple of the oblique convergence and stresses caused by plate bending. For the eastern domain, most nodal planes of earthquakes strike parallel to the outer rise, indicating bending as the prevailing mechanism causing normal faulting. East of 172°W, STS faults strike parallel to the magnetic fabric but oblique (10°–26°) to the axis of the trench. On the basis of a Coulomb failure criterion the trench-oblique strikes probably result from reactivation of crustal faults generated by spreading. INDEX TERMS: 3045 Marine Geology and Geophysics: Seafloor morphology and bottom photography; 7230 Seismology: Seismicity and seismotectonics; 8010 Structural Geology: Fractures and faults; 8150 Tectonophysics: Plate boundary—general (3040); 8164 Tectonophysics: Stresses—crust and lithosphere; KEYWORDS: Aleutian Trench, oblique convergence, stresses, faults Citation: Mortera-Gutie´rrez, C. A., D. W. Scholl, and R. L. Carlson, Fault trends on the seaward slope of the Aleutian Trench: Implications for a laterally changing stress field tied to a westward increase in oblique convergence, J. Geophys. Res., 108(B10), 2477, doi:10.1029/2001JB001433, 2003. 1. Introduction stresses are oriented perpendicular to the bending axis of the subducting plate [Jones et al., 1978; Hanks, 1979]. [2] Normal faults that break the seaward trench slope Other factors noted by Scholl et al. [1982] and Masson (STS) are generally ascribed to the bending of the oceanic [1991] also affect the orientation of STS faults in subduc- plate into the subduction zone [Ludwig et al., 1966; tion zones, for example the inherited fabric of seafloor Parsons and Molnar, 1976; Jones et al., 1978; Scholl et spreading and the obliquity in plate convergence as al., 1982; Hilde, 1983]. STS faults are thus expected to observed along the Chile Trench [von Huene et al., strike parallel to the trench because horizontal tensional 1997]. STS faults are conspicuous along the western sector of the Aleutian Trench, which is obliquely underthrust by Copyright 2003 by the American Geophysical Union. the Pacific plate. Along this sector, the pattern and 0148-0227/03/2001JB001433$09.00 orientation of STS faults are deflected from the expected ETG 6 - 1 ETG 6 - 2 MORTERA-GUTIE´ RREZ ET AL.: FAULT TRENDS AT THE ALEUTIAN TRENCH Figure 1. Tectonic features along the Aleutian STS. Seafloor magnetic anomalies are shaded (labeled with numbers in reference to Cande and Kent [1992] timescale). The regional distribution of earthquakes (1957–1990) with Mw > 3.0 from the USGS hypocenter catalog are marked with crosses. The locations of large earthquakes (1977–1992) with Mw > 4.5 from the Harvard CMT catalog are midpoint lines (oriented to the preferential nodal fault plane) with numbers. Arrows near the trench show the relative motion of the PAC and NAM plates. Maps are in Mercator projection with a standard parallel at 45°N. trench-parallel strike east of the Amlia Fracture Zone relative to the trench is highly oblique (7°–32°), is located (Amlia FZ) and west of the Rat Fracture Zone (Rat FZ). between Stalemate Ridge (169.4°E) to just east of the Rat [3] The Aleutian arc is one of the transitional plate FZ (179°E). The eastern fault-strike domain lies east of boundaries (Figure 1), along which the relative motion of the Rat FZ where the angle of convergence is moderately convergence gradually changes westward from normal to oblique to nearly orthogonal (32°–80°). transform motion [Fitch, 1972; Scotese and Rowley, 1985; [5] This study analyzes the orientation of STS faults DeMets et al., 1990]. Studies of crustal fragmentation along (continuously mapped by GLORIA side-scan imagery transitional margins have concentrated on the shear defor- [Groome et al., 1997]) from 169°E to 165°W (Figure 3) mation of the overriding rather than on the subducting plate and earthquake source mechanisms to model the state of [Kimura, 1986; Geist et al., 1988]. Analyses of the disrup- stress in the upper part of the subducting oceanic litho- tion of the margin of the overriding plate show a strong sphere. We propose a physical model to explain the laterally correlation between changes in convergence angle and changing stress field linked to changes in direction of the tectonic partitioning in the forearc and arc regions [Fitch, relative plate motion along the Aleutian trench in the 1972; Jarrard, 1986; McCaffrey, 1992]. Only a few studies western domain and the observed correlation of the orien- have described evidence for lower plate disruption ascribed tation of Aleutian STS faults with the trends of preexisting to oblique convergence [e.g., PRICO Working Group, 1998; faults in the eastern domain. We cannot differentiate if Dolan and Mann, 1998]. As a consequence of the transition only bending stresses or the laterally changing stress field from convergence into transform motion, it is plausible that reactivates the preexisting faults toward the western half the state of stress in the upper part of the oceanic plate may zone (between 179°E to just west of Amlia FZ) of the deviate from that expected due to pure bending into the eastern domain. subduction zone. [4] The GLORIA (Geological Long Range Inclined Asdic) side-scan sonar survey of the Aleutian STS (Figure 2) 2. Background Information provides an exceptionally revealing data set of imagery [6] The Aleutian Ridge lies on the southern edge of the (Figure 3) to analyze the stress implications of the STS fault Bering Sea and stretches from the Unimak Pass at the pattern in relation to the gradual westward change in Alaska Peninsula to the western end of the Komandorsky relative convergence angle. GLORIA images and focal transform zone. The Aleutian Trench borders the 2200-km- mechanisms of large earthquakes (Mw > 4.5) show patterns long Aleutian Ridge and, except along its far western or of normal faulting on the southern slope of the Aleutian Komandorsky transform sector, tectonically separates the Trench (Figure 4) that are deflected from the expected Pacific (PAC) and North American (NAM) plates (Figure 1). trench-parallel strike. The strike of the deflected fault South of the Aleutian Trench between 165°E and 165°W, pattern can be separated into a western and eastern domain. the main physiographically elements of the STS are the The western domain, where the angle of convergence Stalemate, Rat, and Amlia Fracture Zones. The Stalemate MORTERA-GUTIE´ RREZ ET AL.: FAULT TRENDS AT THE ALEUTIAN TRENCH ETG 6 - 3 Figure 2. GLORIA side-scan sonar survey of the Aleutian Ridge. Lines with arrows and indexed by numbers mark the ship tracks from four R/V Farnella cruises (F287AA, F387AA, F788AA, and F888AA) that collected acoustic images along the Aleutian (gray shaded zone). Map also shows the edges of the 2° Â 3° GLORIA panels (thin grid lines) with their numbers in a corner, the boundaries (thick gray lines) of Figure 3, and the digitized trends (thick dark lines) of main seafloor structures mapped by the sonar. Fracture Zone (Stalemate Ridge) is a zone of age disconti- westward from nearly normal to the trench (79°–84°)at nuity along which early Tertiary seafloor east of the ridge is 165°W to nearly parallel (4°–10°) at 169°E (Figure 1). The separated from late Mesozoic crust to the west [Lonsdale, DeMets and Dixon [1999] solution provides azimuths more 1988]. The north trending but sinuous shape of Stalemate westerly (2°–3°) than the NUVEL-1A azimuths, predicting Ridge reflects pivoting and counterclockwise (CCW) rota- a higher obliquity in the PAC-NAM relative plate motion in tion of the Kula-Pacific spreading center Lonsdale [1988]. the Aleutian western sector. On the contrary, Larson et al. The north-south trend of the Amlia and Rat Fracture Zones [1997] PAC-NAM azimuths are not significant different are nearly perpendicular to the trench axis. Both fracture from the azimuths of NUVEL-1A along the western zones were formed as a result of early Tertiary transform sector.
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