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 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. At the present, NUVEL-1A solution seems to motion between the Kula and PAC plates [Byrne, 1979; Rea be more robust in predicting the relative PAC-NAM motion and Dixon, 1983; Lonsdale, 1988]. along the Aleutian western sector. Using NUVEL-1A [7] Seaward of the Aleutian Trench, a continuous gravity along the Aleutian Trench, the trench-parallel component high (20 to 50 mGal) is associated with an outer trench rise (33 mm yr 1) increases from nearly half of the trench- (Figure 1). The high is best discerned east of the Amlia FZ orthogonal motion at 172 W to equal normal and parallel to the region adjacent to the vicinity of Stalemate Ridge. components at 180 (51 mm yr 1). West of 180 , the trench- Levitt and Sandwell [1995] constructed synthetic gravity parallel motion is larger than the trench-orthogonal compo- profiles across the outer rise to examine the extent of plate nent, increasing from 63 mm yr 1 at the trench intersection bending east of the Rat FZ. Their study indicates that with the Rat FZ to about 74 mm yr 1 at the intersection with lithospheric flexure is uniform along the Aleutian Trench. Stalemate Ridge. They did not attempt to examine the plate flexure west of [9] The Aleutian forearc region has been obliquely the Rat FZ where the gravity field is perturbed by the massif underthrust to the NW by the subducting oceanic crust of the Stalemate Ridge. The seismic profiles of Buffington (Kula or Pacific) since the early Eocene [Engebretson et al., [1973] show a partially flexed STS west of the Rat FZ, 1985; Tarduno and Gee, 1995]. Right-lateral tangential presumably documenting that the bulge of the outer high at plate motion along the western Aleutian ridge has caused least extends westward into the intersection area of the shear deformation of the ridge’s arc massif and produced Stalemate Ridge and the trench. major blocks of arc crust, rotating CW within the right- lateral (dextral) shear couple and also translated westward, 2.1. Plate Convergence and Magnetic Fabric parallel to the arc [Harbert, 1987; Geist et al., 1988; Ryan [8] Along the Aleutian Trench, most recent plate motion and Scholl, 1989]. In the western sector of the Aleutian vectors based on geological data (NUVEL-1A) [DeMets et Ridge (Figure 1), the Near Islands arc segment is primarily al., 1994], global GPS measurements [Larson et al., 1997] in a strike-slip regime with little if any component of an arc- and combining regional GPS data with NUVEL-1A data normal subduction. Strike-slip faulting and extensive shear [DeMets and Dixon, 1999] indicate that the present-day deformation are evident in the Near Islands [Ave´ Lallemant, relative motion of the PAC and NAM plates changes 1996; Ave´ Lallemant and Oldow, 2000]. Marine geophysical ETG 6 - 4 MORTERA-GUTIE´ RREZ ET AL.: FAULT TRENDS AT THE ALEUTIAN TRENCH
Figure 3a. The U.S. EEZ GLORIA side-scan sonar images along the Aleutian STS (in Lambert Conformal Conic projection with standard parallels at 50 N and 59 N). Image of the region between 167 E and 174 E (panels 01–07). The Stalemate FZ is the largest structure imaged by the sonar. observations provide evidence that the far western Aleutian 1988]. Table 1 summarizes the trends of magnetic anomalies Ridge segment is sheared along strike-slip faults that trend and their angles of intersection with the trench. transverse and parallel to the arc trend, such as the Bering- Kresta Trough and Agattu Fault [Scholl et al., 1987; Vallier 2.2. Seismicity et al., 1996]. [11] The distribution of earthquakes and their mecha- [10] Trends of magnetic anomalies south of the Aleutian nisms provide important information about the pattern of Trench (Figure 1) have been most recently mapped and faulting and deformation along the Aleutian STS. Seismic- compiled by Lonsdale [1988] and Atwater and Severinghaus ity maps of the Aleutian Ridge [Frohlich et al., 1982; Taber [1989]. On the basis of the Cande and Kent [1992] geo- et al., 1991; Boyd et al., 1995] show that a large number of magnetic timescale, the anomalies adjacent to the trench date shallow (<40 km depth) earthquakes nucleate in the oceanic from 52 to 41 Ma in the western domain and 65 to 51 Ma in plate. Two sets of earthquake data were used to revise the the central and eastern domains. From east to west the seismic strain field on the southern trench slope: the U.S. anomalies are offset or terminated by the Amlia, Rat, and Geological Survey (USGS) hypocenter catalog of the Aleu- Stalemate Fracture Zones. West of the Rat FZ, the anomalies tian Ridge [Boyd et al., 1995], and the Harvard catalog of rotate CCW to a NE-SW strike (225 –235 ) that is parallel centroid moment tensor (CMT) solutions [Dziewonski and to the abandoned Kula-Pacific spreading ridge [Lonsdale, Woodhouse, 1983]. MORTERA-GUTIE´ RREZ ET AL.: FAULT TRENDS AT THE ALEUTIAN TRENCH ETG 6 - 5
Figure 3b. Image of the region between 174 E and 174 W (panels 08–19). To the west, Buldir Ridge and Rat FZ are the dominant seafloor structures in these images.
[12] Along the Aleutian Ridge in the region between Island are associated with the shift from oblique conver- 176 E and 161 W (which partially covers the western gence to transform motion [Taber et al., 1991]. A distinct sector), Boyd et al. [1995] have optimally relocated earth- seismogenic signature in the focal mechanisms is exhibited quakes with magnitudes greater than 3.0 that occurred by the events occurring along the STS. CMT focal mecha- between 1957 and 1991. Less than 10% of the total number nisms of 16 events along the STS indicate normal faults of earthquakes nucleates south of the trench (Figure 1), with steep dips (Table 2). West of the Rat FZ, eight solutions where the seismicity is confined to the upper part of the ([1] to [8] in Table 2) record nodal planes striking 339 – oceanic lithosphere and is distributed across the expanse of 302 oblique to the trench trend and forming acute angles the STS. of 46 –9 to its axis (Figure 1). In contrast, between the [13] Since 1977, the Harvard CMT catalog documents Rat and Amlia Fracture Zones, four fault planes ([9], three classes of focal mechanisms for the shallow, large [10], [11] and [12] in Table 2) strike subparallel to the earthquakes (<60 km depth with moment magnitudes median strike of the trench (Figure 1). However, east of Mw > 4.5) along the Aleutian Trench: (1) thrusting events and at the Amlia FZ focal planes ([13] to [16] in Table 2) of the interplate zone, (2) strike-slip fault events in the upper do not correlate well with the median trends of the outer plate of the Aleutian Ridge, and (3) normal faulting events rise and flanking trench but strike subparallel to them on the STS of the oceanic plate. North of the Aleutian (Figure 1). The normal earthquake faulting thus shows Trench and east of Rat Island, most earthquakes signal considerable variation in the nodal plane orientation along thrust faulting with the compressional axes oriented orthog- the STS. onal to the convergent plate margin. Slightly west of Rat Island, focal mechanisms associated with thrust faults indicate horizontal components of slip parallel to the ridge 3. Data [Yu et al., 1993]. West of the Near Islands, the slip vectors 3.1. Seismic Reflection Profiles of upper plate earthquakes become aligned with Bering- [14] Evidence of faulting in the STS of the Aleutian Kresta shear zone where their prevailing mechanism is Trench was first recognized by from seismic reflection strike-slip faulting [Newberry et al., 1986; Geist and Scholl, profiles. Faulting of the Aleutian STS from seismic reflec- 1994]. These changes in focal mechanisms west of Rat tion profiles involves a series of step platforms bordered by ETG 6 - 6 MORTERA-GUTIE´ RREZ ET AL.: FAULT TRENDS AT THE ALEUTIAN TRENCH
Figure 3c. Image of the region between 174 W and 165 W (panels 19–27). The N-S trend of Amlia FZ is imaged in GLORIA panels 21 and 22. normal fault escarpments with mostly trench-facing slopes (Figure 4). These profiles show that the Aleutian STS is and thus northward dipping slip surfaces [Hayes and Ewing, offset by a series of normal faults across which the down 1970; Buffington, 1973; Scholl et al., 1982; McCarthy and thrown blocks are systematically toward the trench. The Scholl, 1985]. The GLORIA survey of the U.S. Exclusive number of faults in these profiles is significantly reduced in Economic Zone (EEZ) in 1987 and 1988 digitally recorded the region adjacent to northern Stalemate Ridge, and large approximately 15,000 km of two-channel seismic reflection fault throws (>100 m) occur at distances no greater than data across the Aleutian STS [Hill and McGregor, 1988]. 30 km from the toe of the landward trench slope. Also However, few STS fault parameters can be extracted from different times of active faulting on the STS can be determined these profiles because of their subparallel orientation to the from the EEZ seismic lines where they parallel the trench west trench. of the Rat FZ [Vallier et al., 1996; Mortera-Gutie´rrez, 1996]. [15] In the western domain, some seismic profiles trend The trench-parallel seismic profiles F287AA-33 and perpendicular to the Aleutian Trench. From 167 E to 176 E, F287AA-35 (Figure 5) show two generations of faults two profiles (B30 and B31) from Buffington [1973] and line disrupting the Aleutian STS. Both trench-parallel profiles F287AA-58 from Mortera-Gutie´rrez [1996] provide repre- show that the oceanic basement relief is offset by numerous sentative sections across this segment of the Aleutian STS normal faults. Few of these faults disrupt the sedimentary MORTERA-GUTIE´ RREZ ET AL.: FAULT TRENDS AT THE ALEUTIAN TRENCH ETG 6 - 7
Figure 4. (a) Map of Aleutian STS faults. Thin lines mark fault trends imaged by the GLORIA sonar. Thick midpoint lines show the azimuths of nodal plane from the CMT solutions. Straight lines with labels mark seismic profiles referred in the text. (b) Rose diagrams synthesis the median trends of STS faults for each longitudinal degree interval.
blanket far from the trench (profile F287AA-35). However, 40 km [Laughton,1981;Johnson and Helferty, 1990; profile F287AA-33, which is closer to the trench, shows Somers, 1996]. The sonar swaths are presented in panels faulting of young sediment overlying oceanic basement. of pixel arrays of gray scale squares, where weak returns Whereas in the western half of the eastern domain, three and acoustic shadows are dark, and strong returns are light multichannel seismic profiles (L9-6, L5-7, and L9-12) that as shown in Figure 3. To reduce acoustic distortion, cross the Aleutian Trench (marked in Figure 4) show oceanic GLORIA digital image data are processed [Chavez et al., basement disrupted by normal faults and buried by the 1996] prior to being assembled into large mosaics of turbidite sequence flooding the axis of the Aleutian Trench image panels [Groome et al., 1997]. [Scholl et al., 1982; McCarthy and Scholl, 1985]. Along [17] Figure 3 shows a composite of 23 GLORIA panels profile F387AA-24 (Figure 4), fault throws greater than of the STS. The sediment flooded Aleutian Trench is 100 m are found at distances more than 26 km from the toe imaged as a curving wide zone of homogeneous acoustic of the inner trench slope [Mortera-Gutie´rrez, 1996]. backscatter. East of the Stalemate Ridge (Figure 3a), the trench gradually changes orientation from a NW strike 3.2. Aleutian GLORIA Survey ( 306 ) at 169 E to a WNW strike ( 287 ) at 179 E [16] The GLORIA side-scan sonar system, towed by the (Figure 3b). East of the southernmost reach of the R/V Farnella, regionally imaged the seafloor south of the trench ( 179.5 E), the trench axis changes from an EW Aleutian Ridge (Figure 3), as part of the program to map strike ( 273 ) at 180 to a WSW strike ( 245 ) at 165 W geologically the EEZ of the United States in the 1980s (Figure 3c). North of the trench floor, the lower landward [Hill and McGregor, 1988; Groome et al., 1997]. During trench slope exhibits hummocky patterns that are associ- four cruises in 1987 and 1988 (F287AA, F387AA, ated with the thrust fabric of a wide accretionary prism. F788AA, and F888AA), GLORIA lines imaged about Over the STS physiographic forms of the Stalemate Ridge 500,000 km2 of the Aleutian STS between 165 E and (from 170 E to 175 E), and the Rat (at 178 E) and Amlia 165 W and to a distance of about 200 km seaward of the (at 173 W) Fracture Zones are imaged. The southern trench (Figure 2). The GLORIA side-scan insonifies the trench slope also shows high backscatter contrast where seabed at 6.2–6.8 kHz (100 Hz bandwidth) and at oceanic small-scale features, such as seamounts, fault scarps, and depths (up to 7 km), with a beam width (swath) as wide as abyssal hills are insonified. ETG 6 - 8 MORTERA-GUTIE´ RREZ ET AL.: FAULT TRENDS AT THE ALEUTIAN TRENCH
Table 1. Azimuths and Angles of Tectonic Features Along the Aleutian STSa
Plate Magnetic jm, Nodal fn, Trench Motion fp, Anomaly Magnetic Plane Nodal Azimuth, Direction, Plate Motion, Trends, Anomaly, Strikes, Planes, Mean Fault ff, Mean Longitude deg deg deg deg deg deg deg Strikes, deg Faults, deg Stalemate Ridge 169 E 306 313 7 ...... 300 and 310b 6 and 4 170 E 299 313 14 225 74 ...... 306 and 324b 7 and 25 171 E 298 314 16 225 73 ...... 316 18 172 E 293 314 21 225 68 329 [2] 36 317 24 173 E 296 315 19 226 70 318 [4] 22 316 20 174 E 293 315 22 226 67 318 [2] 25 312 19 175 E 289 316 27 226 63 ...... 313 24 176 E 292 316 24 226 66 ...... 305 13 177 E 287 316 29 234 53 ...... 309 and 270b 22 and 17 178 E 287 317 30 235 52 ...... 304 and 268b 17 and 11
Rat FZ 179 E 279 318 39 233 46 ...... 305 and 266b 26 and 13 180 273 318 45 268 5 ...... 267 6 179 W 267 318 51 268 1 ...... 266 1 178 W 267 319 52 268 1 ...... 265 2 177 W 263 319 56 268 5 294 [1] 31 266 3 176 W 260 320 60 269 9 260 [2] 0 270 10 175 W 260 321 61 270 10 249 [1] 11 269 9 174 W 262 321 59 270 8 ...... 269 7 173 W 263 322 59 270 7 279 [1] 21 268 5
Amlia FZ 172 W 260 322 62 272 12 289 [1] 29 265 5 171 W 255 323 68 273 18 ...... 269 14 170 W 250 324 74 273 23 ...... 270 20 169 W 250 324 74 276 26 ...... 272 22 168 W 248 325 77 277 29 ...... 268 20 167 W 247 326 79 274 27 232 [1] 15 271 24 166 W 247 326 79 275 28 ...... 273 26 165 W 245 327 82 275 30 222 [1] 23 269 24 aEstimates of the directions of PAC-NAM plate motion (NUVEL-1A), trends of magnetic lineations, average strikes of nodal planes in Table 2 (number of CMT solutions in brackets), and mean strikes of SST faults in Table 3. Angles with respect to the trench: fp for the directions of convergence, jm for the magnetic lineations, fn for the earthquake nodal planes, and ff for the STS fault strikes. bBimodal trends.
Table 2. Earthquake Focal CMT Solutions Along the Aleutian STS (1978–1992) b b Depth, Fault Plane Auxiliary Plane Date Longitude Latitude km Maga Strike Dip Strike Dip CMTc Stalemate Ridge 9 Oct. 1989 171.79 E 51.80 N <15 6.0 339 64 174 27 [1] 25 May 1989 172.05 E 51.71 N <15 5.3 319 66 167 27 [2] 9 April 1986 173.24 E 51.05 N <15 5.3 318 62 162 30 [3] 13 Feb. 1988 173.37 E 50.66 N <15 5.2 312 74 101 18 [4] 4 Oct. 1978 173.41 E 50.98 N <15 5.3 313 48 109 45 [5] 7 Feb. 1988 173.45 E 50.76 N <15 6.2 327 48 113 47 [6] 4 Feb. 1988 173.58 E 50.75 N <15 5.0 333 66 274 41 [7] 3 May 1980 173.58 E 51.23 N <15 5.8 302 57 147 36 [8]
Rat FZ 27 July 1984 176.92 W 50.28 N 13 5.8 294 58 245 43 [9] 5 Feb. 1981 176.36 W 50.09 N 22 5.7 262 49 267 41 [10] 15 April 1992 175.98 W 50.21 N 39 5.6 258 49 257 41 [11] 25 May 1988 174.62 W 50.50 N 20 5.7 249 57 268 35 [12] 13 July 1981 173.19 W 50.20 N 16 5.5 279 65 246 29 [13]
Amlia FZ 21 June 1990 172.67 W 50.84 N <15 5.3 289 52 257 42 [14] 4 March 1986 167.04 W 51.52 N 38 5.6 232 49 252 43 [15] 5 June 1981 165.26 W 52.28 N <15 5.5 222 54 285 59 [16] a Magnitude, Mw = 2/3 log Mo 10.7, where Mo is the seismic moment in dyn cm [Hanks and Kanamori, 1979]. bParameters (strike and dip in degrees) of primary and auxiliary nodal planes of focal mechanisms, based on Aki and Richards [1980] convention. cCMT, Harvard catalog [Dziewonski et al., 1994] solutions at the locations and depths reported by Boyd et al. [1995]. MORTERA-GUTIE´ RREZ ET AL.: FAULT TRENDS AT THE ALEUTIAN TRENCH ETG 6 - 9
Figure 5. Representative seismic sections along the Aleutian STS. Both profiles (a) F287AA-33, the closest profile to the trench, and (b) F287AA-35 show normal faults at the surface and overlying sediment layers not disrupted by the older faults imbedded in the oceanic crust.
[18] Trends of STS faults and abyssal hills were digitized [20] West of the Rat FZ, STS faulting is confined within a from the 1:500,000-scale panels and integrated to construct roughly rhomboidal area bordered by the Stalemate Ridge a fault map of the southern trench slope (Figure 4a). Each (panels 04, 06, 08, 09, and 12). Fault density gradually digitized fault is composed of segments with lengths decreases westward across this area. Fault strikes change between 0.9 and 2.4 km. East of the Rat RZ the longest progressively in this direction from about 260 at the Rat FZ faults mapped range from 66 to 157 km in length, but west to about 345 adjacent to the intersection zone between the of this structure the range of maximum lengths is lower, Stalemate Ridge and the trench. Two areas show bimodal 5–88 km. Because seafloor directly beneath the vessel distributions in fault orientations, one at 169 E–170 E along the track line is poorly insonified, the total fault (panel 04) with strikes of 303 and 317 , and the other length is limited to fault segments between two ship tracks. between 177 E and 179 E with more northwesterly strikes Some mapped faults could be longer than shown on the of 268 and 307 . The latter bimodal distribution is the mosaics. For the data analysis, fault azimuths are weighted result of including azimuths from STS faults and abyssal by their length and compiled into 3 bins to construct rose hills on the southern slope of the outer rise. Between 171 E diagrams for each 1 -wide longitudinal corridor of the STS and 176 E, the fault azimuths remain uniform, with fault (Figure 4b). Modal azimuths were determined from these strikes changing eastward from about 316 to about 305 . rose diagrams (Table 3), except where a distinctly bimodal On average the STS faults west of the Rat FZ strike oblique distribution of azimuths is observed thus yielding two to the trench axis, intersecting at an angle of 20 ±7 modal azimuths of fault trends. (Figure 4). 3.3. Images of Fault Patterns 4. STS Fault and Earthquake Patterns [19] Except south of Stalemate Ridge, the GLORIA mosaic shows a clear picture of parallel abyssal hills on [21]Between169 Eand165 W, two prevailing fault the southern slope of the outer rise (Figure 3). The observed trends, NW and E-W, are observed that define the western grain of these hills is similar to that imaged by other side- and eastern fault domains as shown on Figure 6. Faults of scan sonar surveys on the East Pacific Rise [i.e., Cowie et the western domain (west of 179 E) reveal a NW strike that al., 1994]. Between the Rat and Amlia Fracture Zones, is more northerly than the trench by about 20.2 (±3.7 ), trends of the abyssal hills are subparallel to the trench axis. excluding fault strikes of the most westerly interval where On the east side of Amlia FZ, most trends of abyssal hills there are few faults. East of 179 E (eastern domain), where strike oblique to the trench. The southern edge of the trench the trench axis is roughly E-W, STS faults also strike E-W. floor is delineated by lineaments of fault scarps that strike East of the Amlia FZ (173 E), the strike of STS faults both oblique and parallel to the trench axis. remains E-W, but this direction departs from a trench- ETG 6 - 10 MORTERA-GUTIE´ RREZ ET AL.: FAULT TRENDS AT THE ALEUTIAN TRENCH
Table 3. Digitized Seaward Slope Faults at the Aleutian Trench Fault Lengths,c km Fault Strikes,d deg Longitudea Digitizedb Segments Longest Fault Total Average Segment Mean ±1.5 Angle f With Respect to Trench Stalemate Ridge 169 Ee 17 5.3 16.3 0.9 300, 310 6, 4 170 Ee 76 13.8 130.2 1.7 306, 324 7, 25 171 E 235 27.4 414.9 1.7 316 18 172 E 293 26.0 529.5 1.8 317 24 173 E 346 31.6 636.4 1.8 316 20 174 E 354 51.4 547.3 1.5 312 19 175 E 370 41.2 532.1 1.4 313 24 176 E 681 43.5 930. 6 1.4 305 13 177 Ee 909 58.7 1411.2 1.6 270, 309 17, 22 178 Ee 765 87.9 1582.1 2.1 268, 304 11, 17
Rat FZ 179 Ee 731 97.9 1515.5 2.1 266, 305 13, 26 180 873 124.0 1710.5 1.9 267 6 179 W 698 153.6 1449.8 2.1 266 1 178 W 627 102.2 1461.4 2.3 265 2 177 W 618 69.2 952.5 1.5 266 3 176 W 680 102.3 1121.7 1.7 270 10 175 W 707 111.2 1148.0 1.6 269 9 174 W 865 123.8 1283.9 1.5 269 7 173 W 860 66.8 819.1 0.9 268 5
Amlia FZ 172 W 985 156.8 1606.5 1.6 265 5 171 W 749 115.0 1041.1 1.4 269 14 170 W 620 87.2 864.1 1.4 270 20 169 W 528 70.3 729.5 1.4 272 22 168 W 524 68.3 776.2 1.5 268 20 167 W 657 77.5 936.6 1.4 271 24 166 W 589 103.1 935.7 1.6 273 26 165 W 227 37.5 318.6 1.4 269 24 aSelected midpoints for each 1 longitudinal interval at the trench axis. bTotal number of digitized location pairs for all fault segments within each interval. cMeasured lengths of digitized faults in km: maximum length for the longest fault, total length of all faults within each interval, mean distance between points along a fault. dMean fault strike within each interval and f, the angle between the fault trends and trench. eBimodal distribution of fault strikes within an interval.
parallel trend and forms an acute angles <26 with the ENE faults change by 36 as fault trends continuously rotate striking trench axis. The rose diagrams of Figure 4b show from trench-parallel azimuths on the east side of the that a 36 change in the trend of STS faults occurs east and fracture zone to the trench-oblique azimuths on the west west of the southernmost reach of the Aleutian Trench side. ( 179.5 E). A similar change is observed in the nodal plane strikes. 4.1. Western Fault Domain [22] It is important to explain that the change in trend of [23] In the western domain, which begins at 179 E, STS faults between the two domains (Figure 4) is not an just east of the Rat FZ, the strikes of STS faults are artifact of the change in track lines direction at 179 E distinctly more northerly than the trench trend, between (Figure 2). For example, (1) the E-W orientation of abyssal 300 and 317 with a median of 312 (Figure 6). hills on the southern slope of the outer rise is orthogonal to Furthermore, GLORIA seismic profiles (i.e., F287AA-33 the Rat FZ (panels 12 and 13), and this trend does not and F287AA-35 in Figure 5) document two generations deviate from orthogonal regardless of the change in the of STS faults. The younger faulting, which offsets young survey direction; (2) on the northern slope of the outer rise, seafloor sediment, occurred on the STS, whereas the some STS fault scarps (panel 12) are also orthogonal east of older set that only offsets oceanic basement could the Rat FZ where the ship tracks strike diagonal to the correspond to a fault fabric created near a spreading trench, and their azimuths are parallel to those STS faults ridge. In the western domain the trends of STS faults are imaged toward the east where ship tracks are nearly parallel significantly different from the 225 –235 orientation of to the trench; (3) between the Rat and Buldir FZs (panel magnetic anomalies (Table 1). The magnetic anomalies 12), a subtle change in the fault azimuths is shown and the strike northeastward and form angles of 52 –81 with orientations remain orthogonal to the arcuate trend of the the WNW strike of the trench (Figure 1). Most NW Buldir FZ where the ship track bearings are constant; and trending STS faults cross the magnetic lineations at high (4) where no change in survey direction occurs, east of the angles (>69 ), indicating that in the western domain the northern end of Rat FZ (panel 12) the orientations of STS discordance between fault trends and trench axis is MORTERA-GUTIE´ RREZ ET AL.: FAULT TRENDS AT THE ALEUTIAN TRENCH ETG 6 - 11
ones offsetting the oceanic basement, indicating possible activation of older faults. [26] Nodal planes of CMT solutions (Table 2) east of 179 E broadly follow the trench-parallel trend (Figure 6) and suggest that these large normal fault earthquakes are caused by bending stresses. The average difference between trench and nodal plane trends in the eastern fault-strike domain is 9.75 (Table 1). Between 176 W and 175 W, four nodal planes (9, 10, 11, and 12) strike nearly parallel to the median trends of STS faults and the trench (Figure 4). However, east of 175 W the nodal planes (13 and 14) of the eastern domain do not correlate well with the median trends of STS faults. East of Amlia FZ (15 and 16) nodal planes are parallel to the trench and outer rise but strike 39 to 47 more northerly than the STS fault. [27] Along the eastern domain (Figures 1 and 4), where the trench-orthogonal component of oblique convergence is larger than its trench-parallel component, the trend of STS faults is significantly different from the directions of the PAC-NAM plate motion (322 at 179 E to 327 at 165 W) Figure 6. Strikes of faults and nodal planes with respect to (Table 1). E-W fault trends in the far eastern part of the the Aleutian tectonic elements. Symbols stand for the mean eastern domain strike 58 off the directions of convergence, azimuths at each degree interval for the strikes of STS faults where as fault trends closer to the Rat FZ the strike (solid and open circles) and nodal planes (pluses). Lines divergence is at slightly small angles (52 ). indicate the PAC-NAM plate motion (thick solid), the trends of magnetic anomalies (thick dashed), trench (thin dashed) and its average (thin solid). 5. Discussion [28] Both fault patterns and the distribution of large earthquakes along the STS are analyzed in relation to unlikely to be related to the orientation of the seafloor the magnetic anomalies and the gradual increase in oblique spreading fabric. convergence along the plate margin (Figure 6). West of the [24] Eight CMT solutions (Table 2) in this domain have Rat FZ (178 E) the strike of magnetic anomalies along the nodal planes striking oblique to the trench axis (302 –339 STS changes from an E-W to a NE-SW orientation with a median of 320 ) and form acute angles of 9 –46 (Figure 1). West of the southernmost reach of the Aleutian with the trench (Figure 4). However, the trends of these Trench ( 179.5 E) the trench-parallel component of oblique nodal planes are consistent with the strikes of the STS faults convergence becomes larger than the trench-perpendicular (312 –317 ), which form angles with the trench of 19 – component. On the basis of the observed patterns of faulting 24 . The nodal planes are slightly oblique to the NUVEL-1A over the seaward trench slope, we examine the strain- convergence direction (314 –315 ). Along the western stress field by separating the STS region into two domains domain, the STS faults form angles of <13 with the (Figure 7). directions of convergence (Table 1). Far west of 176 E, [29] We test the following two hypotheses: (1) fault most fault trends are thus nearly parallel to the convergence strikes oblique to the trench in a highly oblique convergent vectors (angles <3 ). margin are affected by coupling between the downgoing and overriding plates; and (2) fault strikes parallel to 4.2. Eastern Fault Domain magnetic lineations and oblique to the trench could be [25] East of approximately the Rat FZ (Figure 6), the explained by reactivation of preexisting spreading faults. strike of STS faults in the eastern domains is fundamentally In the first hypothesis, fault strikes in an oblique convergent east-west, varying between 265 and 273 with a median of margin suggest influence of more than one stress field; 269 . In comparison, the trend of the magnetic anomalies perhaps one in which s1 or s2 is parallel to the direction of ranges between 268 and 277 with a median of 271 . The convergence. For this setting, we will examine the con- fabric of STS faults and magnetic anomalies is thus nearly ditions where superposition of convergence and bending parallel (Figures 1 and 4). The faults also strike nearly stresses can explain the observed fault patterns. The latter parallel to the alignment of abyssal hills observed along the hypothesis implies that the relation between stress and southern edge of the GLORIA mosaic (Figure 3) and to the faulting is such that the preexisting faults are suitably magnetic anomalies south of the outer rise axis (Figure 1). oriented with respect to the bending stress. We thus examine Between approximately 179 E and 173 W, the fault direc- here what stress field is compatible with the reactivation of a tions deviate little from the generally E-W trend of the preexisting fabric of faults. trench and outer rise axes, whereas farther eastward angles between the STS faults and the trench gradually change 5.1. Superposed Stress Field from about 10 to about 25 . Further, the seismic profiles [30] A stress model is used to test the notion that the (Figure 5) gathered during the GLORIA survey show that trench-oblique strike of STS normal faults is due to the younger STS faults can not be differentiated from the older dextral shear couple associated with oblique convergence ETG 6 - 12 MORTERA-GUTIE´ RREZ ET AL.: FAULT TRENDS AT THE ALEUTIAN TRENCH
Figure 7. Stress-strain domains along the Aleutian seaward trench slope. (a) Western domain, STS faults trending oblique to the trench. Horizontal stresses (thin arrows) are in reference to the x axis orthogonal to the trench; where f, q, and a are the angles between the convergence direction (thick arrows) and trench, the coordinates of the bending stress and convergence stress field, and the rotation from the superposition of stress fields. In the eastern domain, STS faults are shown striking oblique (j1 to j2) to the trench axis and parallel to the magnetic anomalies (shaded pattern). The coordinate system of both stress fields are shown on (b) a bent lithospheric block and (c) a superimposed dextral shear couple on a bent block. along a transitional plate boundary (Figure 7a), where the stress field to a domain affected by the shear couple owing compression regimen in the STS diminishes by the parallel to oblique convergence. A rotated stress field produced by translation of a forearc sliver. In the example of pure plate superposing large horizontal shear stresses related to the bending, STS faults and focal planes of earthquakes are highly oblique convergence with the bending stresses can expected to be parallel to the trench, and the extension account for normal faults striking oblique to the trench. direction should be perpendicular to the bending axis. The [31] To model the stress field near the trench that is stress model accounts for a gradual transition in the state of deflected by the oblique convergence, we assume the stress along the plate boundary; from a prevailing bending isotropic stress is lithostatic for both bending and conver- MORTERA-GUTIE´ RREZ ET AL.: FAULT TRENDS AT THE ALEUTIAN TRENCH ETG 6 - 13 gence stress fields, and then both stress fields can be s3, is oriented perpendicular to the fault strike, and s1 is considered separately. The trench-parallel orientation of vertical. And also it must have the condition, s3 < s2 < s1 or the bending axis is chosen as the reference coordinate sx < sy < sz, to cause normal faulting. So the components of system (Figure 7a), where z axis is vertical and y and x the stress field to cause normal faulting diagonal to the axes are parallel and perpendicular to the trench, respec- trench are estimated in terms of the superimposed stress tively (Figure 7b). The bending stress field (sB)atthe field: seaward trench slope is considered in terms of a tensor 2 2 composed by a deviatoric stress s3 = sxxB <0, sx ¼ sxxS cos a þ syyS sin a þ sxyS sin 2a 2 3 2 2 sxxB 00 s ¼ s sin a þ s cos a s sin 2a 6 7 y xxS yyS xyS 6 7 6 7 sB ¼ 6 0007; sz ¼ szzS 0 4 5 000 or normalized with the shear stress due to the convergence,
s s s s in reference to x, y, z coordinates system. x ¼ xxS cos2 a þ yyS sin2 a þ xyS sin 2a [32] It is assumed a pure, plane, shear stress field (sC) due sx0y0C sx0y0C sx0y0C sx0y0C s s s s to convergence resulting from a shear couple roughly y ¼ xxS sin2 a þ yyS cos2 a xyS sin 2a parallel to the trench induced by the stress coupling between sx0y0C sx0y0C sx0y0C sx0y0C the forearc and the oceanic plate (plate stress coupling) owing to oblique convergence. The tensor of sC is given by where a is the angle between the faults and the trench, 2 3 0 sx0y0C 1 2 cos 2q 4 5 a ¼ tan 1 sC ¼ ; 2 RBC 2 sin 2q sx0y0C 0 Note the angle a depends on the direction of convergence in reference to x0 and y0 axes that are directed parallel and and the ratio between the bending stresses and stresses due perpendicular to the direction of convergence (q) and z0 axis to the convergence. The angle a also can be used to is vertical (Figure 7c). Then sC is transformed into the evaluate if the shear stresses of the superimposed stress field coordinate system of sB, so its stress components are are large enough to rotate the horizontal stresses due to bending and cause trench-oblique STS faults. sxxC ¼ sx0y0C sin 2q 5.2. Fault Reactivation
syyC ¼ sx0y0C sin 2q [34] Here we also consider the tendency for slip failure along these preexisting faults with respect to (1) the yield sxyC ¼ sx0y0C cos 2q strength of the fault, (2) the fault orientation with respect to the trench (j) and (3) the angle (b) between the normal where q is the angle between the normal to the trench trend to the dipping fault plane and the direction of maximum and the convergence direction. Now the stress field due to principal stress (s1). Considering only bending stresses oblique convergence is superposed with the stress field due acting on the seaward trench slope, Figure 7a shows a to bending, sS = sC + sB, where the components of sS are sketch of the patterns of STS faults trending parallel to the given by magnetic anomalies and oriented oblique to the trench axis to the east (forming a range of acute angles, j1 to sxx S ¼sxx Bþs x0 y0 C sin 2q j2 <30 ). The orientations of the horizontal principal stresses (s2 = sy, s3 = sx) are referenced to the horizontal syyS ¼ sx0y0C sin 2q axis perpendicular to the trench. The state of stress in the normal faulting case is s3 < s2 < s1 (where s1 = sz is sxyS ¼ sx0y0C cos 2q vertical). [35] In Figure 8, a 2-D Mohr circle illustrates that failure or normalizing these components in function of the shear in ‘‘fresh’’ rock is possible for a range of values of stress stresses due to convergence: differences (s1 s3) and f < bf < p/2, where the angle 2bf is defined by the point F on the line AB in which the sxxS=sx0y0C ¼ RBC þ sin 2q maximum shear stress (tmax =[s1 s3]/2) is reached to induce fracture through the rock, the mean stress value syyS=sx0y0C ¼ sin 2q (sm =[s1 + s3]/2) and s1 on the sn axis. The Mohr Circle in Figure 8a shows that shear failure could also occur sxyS=sx0y0C ¼ cos 2q along inherited planes of weakness (for jtj mp sn Sp), instead of forming fresh fractures through the rock when the where RBC = sxxB/sx0y0C is the ratio between the bending Sp values of fractured rocks are less than Su.Iffp for the stresses and the shear stresses due to convergence. preexisting fault planes is less than ff, the failure will take [33] The stress field at the seaward trench slope should place along inclined planes of weakness within a range preserve the condition that the horizontal principal stress, of angles (bE > bp > bG), where bE = 1/2(p + f y), ETG 6 - 14 MORTERA-GUTIE´ RREZ ET AL.: FAULT TRENDS AT THE ALEUTIAN TRENCH
Figure 8. Reactivation of preexisting faults by pure bending. (a) Mohr circle. Dashed lines mark the slopes of the friction law for failure directions in unfractured rocks and in rock with preexisting fractures, see text for term definitions. (b) Geometric relationship of compressive stresses and the fault plane dip, for new fault (bf), preexisting fault trending parallel to the trench (btp), and preexisting fault trending oblique to the trench (bto), where a > d > r, and bf > btp > bto.
1 bG = 1/2(f + y) and y =sin {[sm + Sp cot f/tm]sinf}. (l, m, n) relative to a reference system x, y, z. To find the Thus the possible angle bf for fresh faults is within the maximum shear stress, its value can be solved in terms of 2 range of bp values for preexisting inclined planes of the strike and dip components of the shear stress, Tmax= 2 2 weakness, so slip would initiate along a preexisting plane Tstrike + Tdip. Resolving in terms of the direction cosines of weakness rather than a new fault plane. and the stress differences, the pitch of shear stress in a [36] Although the above fault mechanism includes the plane perpendicular to n^ can be calculated with relationship of the principal stresses and the inclination of () planes of weakness, it does not include the variation of the T n ÀÁðÞs s strength of the material as a function of direction, i.e., the tan q ¼ dip ¼ m2 1 n2 ÀÁz x Tstrike lm sy sx strike of the preexisting faults in relation to sx. The level of failure is then considered in terms of the linear law for the yielding strength of the rock to fail, Tfailure = S0 + ms, The magnitude of shear stress is related to the shear where S0 is considered to vary with direction. Following strength of the rock and the acting stresses: the reasoning of Bott [1959], the maximum shear stress qffiffiffiffiffiffiffiffiffi (Tmax) acting on a fault plane can be expressed in terms of 2 Tmax ¼ S0 þ ms the principal stresses (sx, sy, sz) and the direction cosines MORTERA-GUTIE´ RREZ ET AL.: FAULT TRENDS AT THE ALEUTIAN TRENCH ETG 6 - 15
For normal faulting where S0 6¼ m as function of direction, stresses, tmax/sx0y0C =1/2(s1 s3)/sx0y0C increases as the condition for failure on a preexisting fault is shown in Figure 9b. In turn, this implies a greater ÀÁ compression regime in the seaward trench slope if sx0y0C 2 2 2 T S0 msx þ m sy sx þ n ðÞsz sx is more northerly. On the contrary, the few focal mecha- nisms in the western domain indicate that the compression is more westerly. However, the northerly compression 5.3. Western Domain regime between the PAC plate and the Aleutian forearc [37] In the western region of the Aleutian oblique con- can be diminished by the western translation of a forearc vergent zone, both STS faults and earthquake fault planes sliver due to the oblique convergence [Geist et al., 1988; strike obliquely to the trench by 19 clockwise and fail to McCaffrey, 1992; Ave´ Lallemant, 1996]. Under this con- align parallel to the bending axis of the subducted oceanic dition, the observed fault pattern is more easily explained. plate (Figure 4). West of the Rat FZ (178 E), NE striking Hence we state that the observed trench-oblique faulting magnetic lineations strike 54 –74 oblique to the NW occurs in the western domain because of several unique trending trench and form angles >45 with these STS faults conditions: (1) the translation of forearc sliver by oblique and nodal planes (Figure 6). The 36 change in STS fault convergence reduces the compression along the plate trends west of 178 E suggests that a shear zone with a margin; and (2) the superposition of bending stresses and significant dextral strike-slip motion might affect the upper dextral shear couple owing to oblique convergence part of the oceanic lithosphere in the western domain. between the PAC and NAM plates causes a clockwise Further, the concurrence of the nodal plane and STS fault rotation of the stress field. trends with the directions of plate motion suggests that [42] The magnitude of bending stresses should not differ strike-slip fault mechanisms might dominate along the as much from the eastern region to the western region, as Aleutian STS because of the shear stresses associated with observed from the small differences in the topogravity highly oblique convergence. However, this fails to explain profiles across the Aleutian Trench [Levitt and Sandwell, the tensional axes of eight focal mechanisms of earthquakes 1995]. This strong correlation between the orientations of (with azimuth between 212 and 249 ) between 172 E and STS faults and the inferred fault planes supports the 174 E. In the forearc region, evidence for this wrench hypothesis that the direction of extension along the STS is mechanism can be recognized across the arc segment of deflected by the shear couple attributed to the strength of the Near Islands by strike-slip faulting [Ave´ Lallemant, oblique convergence west of 180 . Consequently, trench- 1996] and oblique-transverse canyons [Geist et al., 1988]. oblique normal faults as well as normal faults associated Masson [1991] speculates that STS fault orientations result with earthquakes result from an extensional stress field that from the deflection of the regional stress field owing to is not oriented parallel to the trench axis. This implies strong dextral shear along the plate boundary. However, effects of interplate coupling on the subduction fault (in reference to dextral shear in the oceanic plate adjacent to these islands the degree of aseismic plate locking along the thrust fault of are not as obvious as one could suggest from the faulting subduction interface between plates) west of 180 that style because all large earthquakes in the seaward trench affects the orientation of the bending stress field acting on slope exhibit normal faulting instead of strike-slip faulting. the uppermost part of the oceanic lithosphere near the [38] For the situation of trench-oblique STS fault strikes, Aleutian Trench. we need to evaluate the angle a of the superimposed stress [43] The tectonic implication of the rotation of the field that could cause trench-oblique STS faults. In the extension direction is that the coupling on the subduction western domain, the median value of the angle between fault between the PAC and NAM plates owing to oblique faults and trench is 19 . Hence a stress field with s3 rotated convergence prevails as some extend over the bending clockwise by 19 from the normal to the trench and s1 extension. The occurrence of the change of fault trends vertical in which s3 < s2 < s1 is called for. For the stress west of 180 where the trench-parallel plate motion is regime affected by the direction of convergence, two larger than its perpendicular component also suggests that situations are considered. changes in the orientation of the horizontal stresses are [39] 1. If the oblique convergence is <45 from the trench related to shear stresses arising from oblique convergence. trend, normal faults are rotated anticlockwise from the At orthogonal-convergence plate boundaries with weakly trench. In the western domain, this is not the situation coupled subduction zones, outer rise earthquakes are because of the observed clockwise orientation of the STS generated only on normal faults striking nearly parallel faults and nodal planes with respect to the trench axis. to the trench, indicating that the state of stress in the [40] 2. In the situation where the oblique convergence is seaward trench slope is mainly controlled by bending or greater than 45 from the trench, the superposed stress fields slab pull forces and is not affected by the state of stress in can explain the STS fault trends in the western domain. the interplate couple zone [Christensen and Ruff, 1988]. [41] However, the fault orientations depend on the angle f However, outer rises adjacent to other trenches not only between the directions of convergence and the trench (f = exhibit tensional events but also compressional events 90 q), and on the stress ratio, RBC = sxxB/sx0y0C (Figure 9). [Christensen and Ruff, 1988], suggesting that compres- For the situation in the western domain, Figure 9a shows the sional stresses in the oceanic lithosphere accumulate near estimated stress field with a stress ratio RBC = 0.6 that the trench where the subduction zone is strongly coupled. is aligned with the observed fault trend for a convergence In the example of the Aleutian subduction zone, compres- angle f =40 . In general as the angle of oblique convergence sional earthquakes do not occur (or have not been (q) increases along the plate margin, RBC decreases, and recorded) along the STS. The variations in the orientations as the ratio of maximum shear stress and convergence of thrust events in the Aleutian forearc region [Yu et al., ETG 6 - 16 MORTERA-GUTIE´ RREZ ET AL.: FAULT TRENDS AT THE ALEUTIAN TRENCH
Figure 9. Superimposition of stresses related to the STS fault trends in the Aleutian western domain. (a) Estimate of the relative stresses shown in function of the azimuth of normal faults, under fixed conditions, RBC = 0.6, q =50 , and f =40 . The shaded zone marks the range of observed angles of the STS fault trends with the trench axis. (b) Ratios of the maximum shear stress (gray line) and bending stress (dark line) with the stress due to convergence are shown in function of the angles of oblique convergence.
1993] and the documented shearing in the Near Island arc and also there is a mayor coupling for a strike-slip motion segment [Ave´ Lallemant, 1996] suggest that the stress field than for a dip-slip motion of a highly oblique convergent between the trench and arc is affected by oblique subduc- plate boundary. Further the interplate stress coupling should tion. One possible explanation is that the stress field in the increase for more oblique convergence, given the negative forearc region is redirected by the prevailing interplate buoyancy of the slab for a lesser dip-slip motion. If dextral shear couple between the PAC descending slab and horizontal bending stresses in the oceanic plate are redi- the NAM upper plate wedge, as the obliquity in conver- rected by the stress field associated with the oblique gence increases to the west. Because of this interplate convergence, the magnitude of the stresses due to the stress coupling, the massif of the island arc is torn and dextral shear couple can be large enough to rotate the sheared into several forearc slivers and arc blocks, as extension axis, from trench-perpendicular to an orientation suggested by Geist et al. [1988]. oblique to the trench. [44] The situation in the western domain demonstrates that trench-oblique faults and nodal planes provide evidence 5.4. Eastern Domain that the stress field in the oceanic plate is affected by the [45] The parallel trends of STS faults and magnetic interplate stress coupling owing to oblique convergence, lineations suggest that old spreading faults have been MORTERA-GUTIE´ RREZ ET AL.: FAULT TRENDS AT THE ALEUTIAN TRENCH ETG 6 - 17 reactivated in eastern fault-strike domain (Figure 7a). On the slopes of spreading ridges, patterns of newly formed faults tend to strike nearly parallel to regional patterns of magnetic anomalies [i.e., Menard, 1967; MacDonald, 1982]. Side-scan sonar surveys have provided new insights on fault-generated morphology and abyssal hill formation [i.e., Goff et al., 1993; Cormier and MacDonald, 1994]. The topography of a spreading ridge is predominantly shaped by parallel, inward facing normal faults (rooted at depths <5 km) that reflect the stresses associated with spreading mechanism in the oceanic crust [Thatcher and Hill, 1995]. At some distance from the ridge, these spreading faults become healed by hydrothermal alteration associated with the aging of the oceanic crust [i.e., Lister, 1980]. These parallel, fault-bounded blocks at the ridge give rise to abyssal hills observed in the older ocean basins. Thatcher and Hill [1995] proposed that normal faults at fast spreading centers (such as the Kula-Pacific spreading center) are formed with dips from 45 to 60 near the ridge; later they are rotated by <5 after cooling (with 40 –55 dips). As the spreading fabric approaches the STS, the faults are tectonically rotated 10 to 15 by bending near the trench, resulting in a dip of preexisting faults with a range of 50 –70 at the seaward trench slope. Mortera-Gutie´rrez [1996] quantified the trenchward dips Figure 10. Stresses and strength with respect to the of outer rise nodal planes in the central and eastern orientation of preexisting faults. Solid and dashed curves domains that vary from 49 to 78 . These values seem represent a series of estimates respectively for the applied to be within the range of expected trenchward dipping stresses and strength depending on the angle between the faults along the STS that were created at the southern limb directions of dip (q) and the horizontal minimum stress (s ). of the Kula-Pacific spreading center. This suggests that if 3 The estimates are computed for a normal fault, dipping 45 old fault planes existed with trenchward dips between 50 with a r = 0.12, and varying S and R =(s s )/(s s3). and 70 along the STS, then the slip would occur on those o 1 2 1 The shaded zone marks a minimum in shear strength. planes of weakness rather than produce a slip across a new fault plane. [46] For preexisting faults near the trench at angles <30 with the bending axis, the angles bto of trench-oblique the material as a function of the angle (q) between the fault planes should have less value than the angles btp of direction of dip and the sx direction. Varying So and R = trench-parallel faults having the same true dip (Figure 8b). (sy sz)/(sx sz), a minimum in the shear strength of If this is the situation, the dips observed by Mortera- the fault plane occurs when the applied stresses reach a Gutie´rrez [1996] could be used to estimate bto angles for maximum in a range of orientations 20 < q <20 . This trench-oblique preexisting faults in relation to the strike shows that the normal faulting preferentially slips along (j) of the magnetic anomalies with the trench, using the the trench-oblique STS faults. relation: tanr =tand cosj, where r and d are the apparent [48] Nodal planes of STS earthquakes (Table 2) are and true fault dips with respect to the direction of sx. The either oriented nearly parallel to the faults at the surface apparent dip of the trench-oblique faults is in the direction or parallel to the trench axis in both, the western and of sx which is perpendicular to the outer rise axis. To the eastern halves of the eastern domain. This suggests that east of Amlia FZ, the angles j at 171 W and 165 W range bending stresses may either induce slipping on trench- from 18 to 30 , respectively. Assuming these preexisting oblique preexisting faults in the uppermost part of the faults have a constant true dip of d =65 , their r at 171 W oceanic lithosphere (<10 km depth), or on ‘‘fresh’’ trench- and 165 W are estimated 64 and 62 ;andtheir parallel faults at greater depths into the brittle oceanic corresponding values of bto are 64 and 62 . Furthermore, lithosphere (10–40 km depth). Christensen and Ruff normal faults embedded in unfractured gabbro can initiate [1988] observed that the largest normal fault earthquakes at angles bf =61 [Jaeger and Cook, 1979]. This angle is on the Aleutian STS are consistently parallel to the trench. within the range of values for slip along the inherited The significant differences in orientations shown in Table 1 planes of planes associated with the trench-oblique STS between the faults imaged on GLORIA sonar images and faults in the eastern domain. the inferred fault planes from some earthquake solutions [47] The minimum shear strength of the preexisting suggests that the faulting mechanism in the eastern domain normal faults in the eastern domain is within the range is partitioned with depth in the upper oceanic lithosphere of observed fault orientations (18 < j <30 ). Utilizing as it bends downward into the subduction zone. East of Bott [1959] relations for a preexisting fault plane inclined the Amlia FZ, fault planes of two CMT earthquakes at 45 with r =(sx/sz) = 0.12, Figure 10 shows the (15 and 16) are considerably more concordant with the strike variation of the applied stresses and the shear strength of of the trench axis than with the strikes of STS faults ETG 6 - 18 MORTERA-GUTIE´ RREZ ET AL.: FAULT TRENDS AT THE ALEUTIAN TRENCH
(Figure 4) and magnetic anomalies (Figure 1). One possible and Amlia Fracture Zones and oblique to the trench east of explanation for the alignment of nodal planes with the the Amlia FZ. trench is that bending stresses have caused new trench- [52] In the western domain west of the Rat FZ (169 E– parallel faults to form below the fractured upper layer that 178 E), fault trend deflection is best explained by an parallels the fabric of seafloor spreading. Where the extensional mechanism that results from superposing orientations of earthquake nodal planes are nearly parallel stresses related to plate bending and increasing interplate to the STS faults and magnetic fabric, the reactivation of shear coupling along this transitional convergent-to-trans- preexisting spreading faults by bending occurs near the form plate boundary. West of the trench’s southern most seafloor surface. deflection (near 180 ), the strikes of STS faults change from [49] However, the normal faults from earthquake mech- E-W to 36 more to the northwest and thus oblique to the anisms and a large degree of seafloor roughness between trench where the trench-parallel slip gets larger than the the trench and outer rise along the western half ( 178 E trench-orthogonal component of oblique convergence. An to 173 W) of the eastern domain could be caused either analytical model of the superposed stresses suggests that the by extension due to pure bending or by a horizontal stress extensional, horizontal stresses near the trench are rotated field oriented oblique to the trench, resulting from oblique and reoriented relative to the bending axis as oblique convergence. Furthermore, the similarity of trench-parallel convergence increases to the west, causing faults to strike trends in the western half makes it difficult to determine oblique to the trench. The geological evidence of strike slip whether the state of stress near the trench is different faulting and oblique underthrusting earthquakes in the from that expected from pure bending of the oceanic western sector of the Aleutian Ridge (besides enhancing lithosphere. If shear stresses due to the increase in the rate and extent of fragmentation of the Aleutian Ridge) obliquity are not large enough to rotate the in situ supports that the level of interplate coupling owing to the stresses, then new faults or preexisting faults are activated oblique convergence prevails toward the west and causes parallel to the trench by the bending mechanism. How- the stress field rotation along the Aleutian STS. ever, if shear stresses attributed to oblique convergence [53] For the eastern domain, all preexisting faults (parallel are sufficiently large, the state of stress in the western half to the magnetic fabric) are reactivated along the STS. In its could either cause trench-oblique faulting or reactivate western half between Rat FZ and Amlia FZ, bending preexisting trench-parallel faults. In a small area east of stresses are normal to the preexisting faults, where STS the Rat FZ (178 E–179 E), a small number of faults faults strike parallel to the bending axis. It is difficult to strike oblique to the trench along trends significantly discern whether the STS faults are either strongly controlled different from the trench-parallel faults to the east and by bending or by stresses arising from oblique convergence. from the fabric of magnetic anomalies of the STS. The In its eastern half east of the Amlia FZ, bending stresses are fault pattern in this small area indicates that the shear oblique to the preexisting faults, where the direction of stress due to the increase in oblique convergence initially convergence is nearly orthogonal. In this half, bending is the affects the fault trends along the STS in the western prevailing cause of active normal faulting in the upper corner of the eastern domain. oceanic lithosphere, but it is not enough to induce new faults striking parallel to the trench. Instead bending reac- tivates the trench-oblique preexisting faults. Along this half, 6. Conclusions the trends of some nodal planes strike subparallel to the [50] The southern or seaward trench slope (STS) border- bending axis and oblique to the preexisting faults. Thus this ing the Aleutian Trench and adjacent arc of the Aleutian situation in dual orientations in fault strikes indicates that Ridge forms part of a transitional plate boundary along faulting in the upper oceanic lithosphere is partitioned with which the relative angle of convergence decreases gradually depth; at shallow depths <10 km, the orientations of westward, where a strong correlation between changes in preexisting faults control the strikes of STS faults, whereas convergence angle and tectonic partitioning is observed in at greater depths (10–40 km) the fault trends are controlled the forearc and arc regions. The swath imaging GLORIA only by bending. survey of the Aleutian STS provides a laterally continuous and coherent data set implying that the oceanic lithosphere [54] Acknowledgments. Mortera-Gutie´rrez is grateful for the finan- cial support given by the American Geological Institute, CONOCO, and is deformed near the trench by a state of stress attributable USGS. This research was supported by the grants of NSF-JOIDES, to the plate bending and the gradual westward increase in CONACyT (R125709-T and R34906-T), and UNAM-DGAPA oblique convergence. (IN110897, IN104199 and IN114602). Our greatest thanks for their com- ments to T. Vallier, T. Hilde, W. Sager, S. Uyeda, and J. Freymueller. [51] The GLORIA data set documents that fracturing of the Aleutian STS is dominated by long linear sets of fault References scarps, and exhibit E-W trends of seafloor relief (spreading Aki, K., and P. G. Richards, Quantitative Seismology, vol. 1, 557 pp., W. H. fabric) south of the outer rise axis. The regional trends of Freeman, New York, 1980. STS fault scarp thus define two fault-strike domains, a Atwater, T., and J. Severinghaus, Tectonic maps of the northeast Pacific, in western one of NNW strikes, and an eastern domain of The Geology of North America, vol. N, The Eastern Pacific Ocean and Hawaii, edited by E. L. Winterer, D. M. Hussong, and R. W. 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