Subduction Of Aseismic Ridges Research Articles

The Andes

Dr C. M. Bell has provided the following Introduction to the published papers: The Andes have been described as typical of an orogenic belt produced by subduction of an oceanic plate beneath a continental margin. Models of Andean geological evolution are, however, commonly poorly constrained due to the limited amount of data available from this very large area. Much research has been directed towards specific problems such as the magmatic rocks, but knowledge and understanding of many other aspects of Andean geology are still in their infancy. The purpose of the meeting of 5 October was to encourage geologists currently working in the Andes to provide up-to-date accounts of research in progress. Consequently the resulting collection of papers covers a wide range of topics and geographical areas. A major problem of Andean geology is the relative inaccessibility of much of the published data. This results both from language barriers and the limited distribution of some South American publications. One of the aims of this set of papers is to provide access to aspects of this widely scattered and somewhat obscure literature. The authors of the first two papers both consider aspects of Andean subduction. Wortel highlights the fact that despite a relatively constant convergence rate along the length of South America the late Cenozoic geology shows distinct variations in seismicity, tectonics and magmatic activity. He considers that the main factor controlling these variations is the age of subducted lithosphere. A lesser factor is the subduction of aseismic ridges and seamount

Subduction of aseismic ridges beneath the Caribbean Plate: Implications for the tectonics and seismic potential of the northeastern Caribbean

Normal seafloor entering the Puerto Rico and northern Lesser Antillean trenches in the northeastern Caribbean is interrupted by a series of aseismic ridges on the North and South American plates. These topographic features lie close to the expected trend of fracture zones created about 80–110 m.y. ago when this seafloor was formed at the Mid‐Atlantic Ridge. The northernmost of the ridges that interact with the Lesser Antillean subduction zone, the Barracuda Ridge, intersects the arc in a region of high seismic activity. Some of this seismicity including a large shock in 1974, occurs within the overthrust plate and may be related to the deformation of the Caribbean plate as it overrides the ridge. A large bathymetric high, the Main Ridge, is oriented obliquely to the Puerto Rico trench and intersects the subduction zone north of the Virgin Islands in another cluster of seismic activity along the inner wall of the trench. Data from a seismic network in the northeastern Caribbean indicate that this intersection is also characterized by both interpolate and intraplate seismic activity. Magnetic anomalies, bathymetric trends, and the pattern of deformed sediments on the inner wall of the trench strongly suggest that the Main and Barracuda ridges are parts of a formerly continuous aseismic ridge, a segment of which has recently been overridden by the Caribbean plate. Reconstruction of mid‐Miocene to Recent plate motions also suggest that at least two aseismic ridges, and possibly fragments of the Bahama Platform, have interacted with the subduction zone in the northeastern Caribbean. The introduction of these narrow segments of anomalous seafloor into the subduction zone has segmented the arc into elements about 200 km long. These ridges may act as tectonic barriers or asperities during the rupture processes involved in large earthquakes. They also leave a geologic imprint on segments of the arc with which they have interacted. A 50‐km landward jump of the locus of island arc volcanism occurred in Late Miocene time along the northern half of the Lesser Antilles. We postulate that the subduction of a segment of seafloor of anomolously thick crust, being more buoyant than adjacent seafloor, resulted in a marked shoaling in the dip of the descending slab and, therefore, a shift of the locus of volcanism. In the region near western Puerto Rico and eastern Hispanolia, Plio‐Pleistocene interaction with a similar feature, in this case a part of the Bahama Platform, about 3–4 m.y. ago led to a jump in the locus of subduction as evidenced by a gap in the downgoing seismic zone. That segment of the Bahama Platform interferred with the subduction process and was subsequently sutured onto the Caribbean plate when the boundary jumped about 60 km to the northeast. The maximum size of historic shallow earthquakes along the Lesser Antillean arc varies from about 7.0–7.5 in the center of the arc where the dip of the shallow part of the plate boundary is steep to 8.0–8.5 along the northern part of the arc where the dip is shallow. The interaction of anomalous seafloor, as along the northern Lesser Antilles, can lead to the development of a wider than normal zone of interplate contact and hence to earthquakes that are larger than those associated with more typical seafloor entering subduction zones. Major seismic gaps and regions of high seismic potential currently exist along the northern Lesser Antilles and to the north of Puerto Rico. Both gaps are bounded by anomalous features on the downgoing plate. The intersection of these features with the plate boundary created large asperities that may be good places to search for precursors to future large earthquakes. A great shock in 1787 may have ruptured an existing seismic gap north of Puerto Rico between 65° and 67°W. Thus that gap can be expected to eventually rupture again in a great shock and not to accommodate plate motion by totally aseismic processes.

Subduction of aseismic ridges beneath the Caribbean Plate: implications for the tectonics and seismic potential of the northeastern Carribean : McCann, W.R. and L.R. Sykes, 1984. J. geophys. Res., 89(B6):4493–4519

Subduction of aseismic ridges beneath the Caribbean Plate: implications for the tectonics and seismic potential of the northeastern Carribean : McCann, W.R. and L.R. Sykes, 1984. J. geophys. Res., 89(B6):4493–4519

Controls of subduction geometry, location of magmatic arcs, and tectonics of arc and back-arc regions

Most variation in geometry and angle of inclination of subducted oceanic lithosphere is caused by four interdependent factors. Combinations of (1) rapid absolute upper-plate motion toward the trench and active overriding of the subducted plate, (2) rapid relative plate convergence, and (3) subduction of intraplate island-seamount chains, aseismic ridges, and oceanic plateaus (anomalously low-density oceanic lithosphere) cause low-angle subduction. Under conditions of low-angle subduction, the upper surface of the subducted plate is in contact with the base of the overlying plate, the wedge of low-density asthenosphere is replaced by subducted lithosphere, and the width of the arc-trench gap either is significantly increased or a magmatic arc is not developed within the overlying plate. The fourth factor is age of the subducting lithosphere. Subduction of young lithosphere produces two opposing tendencies: (1) low-angle subduction and increased arc-trench distance, owing to its low density; and (2) decreased arc-trench distance, owing to its higher temperature. Two factors of secondary importance contribute to variation in subduction-zone geometry and arc-trench distance. Accretion of sediment in trenches depresses the upper portion of the subducting oceanic plate and causes the trench axis to migrate seaward. Prolonged subduction thickens the upper plate, depresses the isotherms in the subducted plate, and may create a broader arc. Both factors increase the arc-trench gap. The four primary factors also control development of other tectonic elements, such as regional subsidence (for example, the Amazon basin and a portion of the Cretaceous Interior Seaway of western United States), intra-arc extension (for example, the Basin and Range province), foreland fold and thrust belts, and Laramide-style tectonics.

A mechanical model for plate deformation associated with aseismic ridge subduction in the new hebrides arc

Tectonic features associated with a subducting fracture zone-aseismic ridge system in the New Hebrides island arc are investigated. Several notable features including a discontinuity of the trench, peculiar locations of two major islands (Santo and Malekula), regional uplift, and the formation of a basin are interpreted as a result of the subduction of a buoyant ridge system. The islands of Santo and Malekula are probably formed from an uplifted mid-slope basement high while the interarc basin of this particular arc is probably a subsiding basin instead of a basin formed by backarc opening. The situation can be modeled by using a thin elastic half plate overlying a quarter fluid space with a vertical upward loading applied at the plate edge. This model is consistent with topographic and geophysical data. This study suggests that subduction of aseismic ridges can have significant effects on tectonic features at consuming plate boundaries.

Subduction process of a fracture zone and aseismic ridges -- the focal mechanism and source characteristics of the New Hebrides earthquake of 1969 January 19 and some related events

The subduction of the D'Entrecasteaux fracture zone-seismic ridge system in the New Hebrides island arc is investigated on the basis of the focal process of the New Hebrides earthquake of 1969 January 19 (m_b= 6.4, h= 107 km), mechanisms of some related events, seismicity and regional tectonics. A notable feature of this island arc is the discontinuity of the New Hebrides Trench in the central New Hebrides where the ridge-fracture zone is subducting and intersecting the arc. The 1969 New Hebrides earthquake occurred along the subducted portion of the fracture zone and is characterized by unusual waveforms with remarkably large excitation of long period waves, rare for earthquakes of comparable mb and depth; the P-wave records show anomalously long duration and complexity indicating that the earthquake was a multiple event with a source duration of at least 30 s. P-wave first motions, seismicity data and synthetic body-wave seismogram indicate that the earthquake represents transverse left-lateral motion on a nearly vertical, E—W trending fault plane with a slip vector subparallel to the down-dip direction of the Benioff zone. The total seismic moment is found to be 5 × 10^(26) dyne cm by matching the amplitudes and waveforms between observed and synthetic surface waves. The location and mechanism of this earthquake suggest that the D'Entrecasteaux fracture zone structurally extends to the east of the trench. This structural boundary at depth seems to be reflected in the spatial distribution of two earthquake swarms which are bounded sharply at the latitude of 15.2°S. At the extension of the ridge-fracture zone, the activity of intermediate depth earthquakes, which are characterized by a very consistent pattern of down-dip extensional mechanism, is much higher and their depths are systematically shallower than in the adjacent regions. These features can be interpreted as a consequence of subduction of a buoyant ridge and the resultant increase in the extensional stress at the intermediate depths of the sinking slab. Fault-plane solutions of 22 earthquakes suggest that the subduction of aseismic ridges in the New Hebrides is characterized by high-angle thrusts. The lithospheres on the two sides of the D'Entrecasteaux fracture zone under the arc subduct more or less independently and generate alternating left-lateral and right-lateral earthquakes along the subducted portion of the fracture zone.

COLLISION OF THE IZU-BONIN ARC WITH CENTRAL HONSHU: CENOZOIC TECTONICS OF THE FOSSA MAGNA, JAPAN

The Izu-Bonin arc joins with Honshu at the Fossa Magna where pre-Miocene terrains bend in a cusp form. The Miocene terrains in the region also have a northward-convex structure north of the Izu Peninsula. Moreover, highly compressive deformation, Quaternary strong uplift and anomalous trajectories of crustal stress axes also characterize this region.These features of central Honshu at the junction are explained well by assuming that a north-south trending plate boundary has been located off central Honshu since the late Cretaceous. The bend of the terrains was formed for the most part in the early Tertiary by buoyant subduction of aseismic ridges lying along the north-south trending transform fault.The Izu-Bonin arc, which was developed along this transform fault, has been dragged northward by oblique subduction of the Pacific plate and underwent subduction beneath central Honshu during the late Tertiary. In the early Quaternary, the Izu Block (the Izu Peninsula) of the Izu-Bonin arc collided with central Honshu and is pushing it north-northwestward. It is very likely that the triple junction off central Honshu has been located at its present position relative to Honshu since the late Mesozoic.