Seismicity and tectonics of the far Western Aleutian Islands

Newberry, James T.; Laclair, David L.; Fujita, Kazuya

doi:10.1016/0264-3707(86)90030-x

Cited by 28 publications

(9 citation statements)

References 27 publications

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“…Slightly west of Rat Island, focal mechanisms associated with thrust faults indicate horizontal components of slip parallel to the ridge [ Yu et al , 1993]. West of the Near Islands, the slip vectors of upper plate earthquakes become aligned with Bering‐Kresta shear zone where their prevailing mechanism is strike‐slip faulting [ Newberry et al , 1986; Geist and Scholl , 1994]. These changes in focal mechanisms west of Rat Island are associated with the shift from oblique convergence to transform motion [ Taber et al , 1991].…”

Section: Background Informationmentioning

confidence: 99%

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

2003

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[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°) to the 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. Implications for a laterally changing stress field tied to a westward increase in oblique convergence,

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Section: Background Informationmentioning

confidence: 99%

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

2003

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“…1). The general seismic, structural, and magmatic character of the arc in this region led Newberry et al (1986) to conclude that it was not underlain by subducting oceanic crust. More recently, Boyd and Creager (1991) have imaged the aseismic extension of a subducting slab beneath all of the western Aleutian arc.…”

Section: Magmatic and Tectonic Development Of The Komandorsky Regionmentioning

confidence: 99%

Magnesian andesite in the western Aleutian Komandorsky region: Implications for slab melting and processes in the mantle wedge

Yogodzinski

Kay

Volynets

et al. 1995

Geological Society of America Bulletin

541

219

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The role of the subducting lithospheric slab in the genesis of mantle-derived (primitive) magmas is investigated through a study of volcanic rocks formed in the tectonically strike-slip-dominated western Aleutian arc. Two types of chemically and petrologically distinctive primitive andesites have been found among the Miocenelate Pleistocene-age volcanic rocks in the western Aleutians. These are termed the ''Adak-type'' and ''Piip-type'' magnesian andesites. Trace element and isotopic characteristics indicate that Adak-type magnesian andesites (adakites) formed principally as small percentage melts of the basaltic portion of the subducting oceanic crust, leaving a clinopyroxene-garnet-rutile residual mineralogy. The resulting slab melt signature (high La/Yb, Sr) distinguishes Adak-type magnesian andesites from all other Aleutian volcanic rocks. Primitive characteristics (high Mg#, Cr, Ni) and intermediate compositions (ϳ59% SiO 2) of Adak-type magnesian andesites were acquired by interaction with peridotite and/or basalt in the mantle wedge. The absence of olivine phenocrysts from Adak-type magnesian andesites indicates that they were not equilibrated with peridotite and so are unlike Piip-type magnesian andesites, which appear to have equilibrated under low pressure and hydrous conditions in the subarc mantle. Piip-type magnesian andesites also contain a slab melt component, but reaction-equilibration with peridotite has lowered La/Yb and Sr to levels like those of common Aleutian volcanic rocks. Miocene-age calc-alkaline rocks of the Komandorsky Islands have chemical characteristics transitional between those of Adak-type magnesian andesites and common Aleutian volcanic rocks from the central and eastern arc. In a source mixture of depleted mantle wedge, slab melt, and sediment, the Komandorsky rocks have a relatively large contribution from the slab melt endmember. The strong slab melt signature among western Aleutian rocks is attributed to highly oblique convergence that produced a slow subduction path into the subarc mantle. Geochemically, the slab melt provided a high Sr, La/Yb, La/Ta, and low Ti/Hf endmember to the western Aleutian source mixture. The enhanced role for slab melting in the western Aleutians may be like that predicted for Archean systems and for modern systems where the subduction zone is warm. In this regard, Adak-type magnesian andesites are probably the appropriate analog to sanukitoids and other primitive andesitic rocks of Archean age.

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“…Of the remaining three events, two (A9,A21) are located in the far western portion of the arc, near the Kamchatka peninsula, yet one of these (A9) occurred only 4 days after the 1965 Rat Islands event, and the other (A1) occurred in 1929 following an earlier sequence of underthrusting events in the early 1900s. The two events (A9,A21) which are located at the intersection of the Aleutian and Kurile Islands trenches, near the Komandorsky Islands, may be rela•ed to a more complicated tectonic regime [see Newberry et al, 1986]. The locations of the outer rise events along with the aftershock areas of the recent great underthrusting events are shown in Figure 10.…”

Section: Aleutian-alaska Arc Regionmentioning

confidence: 99%

Seismic Coupling and Outer Rise Earthquakes

1988

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Variation of interplate seismic coupling at subduction zones is a major factor controlling the size of the largest underthrusting events. This variation also has a profound effect on the regional intraplate stresses in the vicinity of the subduction zone. Outer rise seismicity is strongly correlated with variations in interplate coupling, reflecting the stress state of the interplate coupled zone. Over 200 outer rise earthquakes with known focal mechanisms are used to investigate the relationship between stresses in the outer rise and interplate seismic coupling. These events occur within the downgoing (i.e., oceanic) plate near the bathymetric trench axes and generally fall into the categories of tensional (normal) or compressional (thrust) with their tensional or compressional stress axes oriented approximately horizontal and perpendicular to the trench. In uncoupled subduction zones, only tensional outer rise earthquakes occur, which indicates that the outer rise is dominated by tensional stresses associated with plate bending and/or slab pull forces. In strongly coupled subduction zones, both tensional and compressional outer rise events are found. These events are related both spatially and temporally to the distribution of large underthrusting earthquakes and are thus an integral part of the earthquake cycle. In the strongly coupled regions, tensional outer rise events follow large underthrusting events as the outer rise is temporarily in tension due to the underthrusting motion. Compressional outer rise events take place as compressional stress slowly accumulates oceanward of locked sections of the interplate zone. In four instances, compressional outer rise earthquakes have been followed by large underthrusting events which have occurred 2, 4, 7, and 19 years after the associated outer rise event. The remaining compressional outer rise events are located in regions that are either known seismic gaps or in regions where the seismic potential is unknown. The occurrence of compressional outer rise earthquakes suggests that compressional stress is accumulating in the adjacent interplate region and that there is the potential for a future large underthrusting event in the region. Thirty compressional outer rise events have been located in trench segments of Middle and South America, the Kurile Islands, the Tonga and Kermadec islands, the New Hebrides Arc, and the Solomon Islands regions. In both the southern Kamchatka and northern New Hebrides regions the outer rise seismicity indicates that the stress regimes in the outer rise have changed with time from tensional, following a previous large underthrusting event, to compressional at present. Thus three stages of the cycle from underthrusting to tensional outer rise regime to compressional outer rise regime are present, requiring only the occurrence of the next underthrusting event to complete the cycle. The occurrence of compressional outer rise events is useful for assessing the seismic potential of a region on an intermediate time scale.

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Seismicity and tectonics of the far Western Aleutian Islands

Cited by 28 publications

References 27 publications

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

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

Magnesian andesite in the western Aleutian Komandorsky region: Implications for slab melting and processes in the mantle wedge

Seismic Coupling and Outer Rise Earthquakes

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